diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/dynamicprogramming/LongestSubstringWithoutRepeatingCharacters.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/dynamicprogramming/LongestSubstringWithoutRepeatingCharacters.java
new file mode 100644
index 0000000000000000000000000000000000000000..b2a53f40d95d022e75ccc2f9dbf4c871039e350e
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/dynamicprogramming/LongestSubstringWithoutRepeatingCharacters.java
@@ -0,0 +1,28 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.dynamicprogramming;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class LongestSubstringWithoutRepeatingCharacters {
+    public int lengthOfLongestSubstring(String s) {
+        if (s.length() == 0) {
+            return 0;
+        }
+
+        // Character as key and index as value
+        Map<Character, Integer> map = new HashMap<>();
+        // two pointers
+        int l = 0;
+        int max = 1;
+        for (int r = 0; r < s.length(); r++) {
+            if (map.containsKey(s.charAt(r))) {
+                // Math.max and +1 is important
+                l = Math.max(l, map.get(s.charAt(r)) + 1);
+            }
+            map.put(s.charAt(r), r);
+            max = Math.max(max, r - l + 1);
+        }
+        return max;
+    }
+}
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/AddAndSearchWordDataStructureDesign.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/AddAndSearchWordDataStructureDesign.java
new file mode 100644
index 0000000000000000000000000000000000000000..c477877f00293f9a21558bb355b280b761888cb5
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/AddAndSearchWordDataStructureDesign.java
@@ -0,0 +1,81 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+import java.util.ArrayList;
+import java.util.List;
+
+class AddAndSearchWordDataStructureDesign {
+    private Character ch = null;
+    private AddAndSearchWordDataStructureDesign[] children =
+            new AddAndSearchWordDataStructureDesign[26];
+    private boolean isEnd = false;
+
+    /** Initialize your data structure here. */
+    public AddAndSearchWordDataStructureDesign() {}
+
+    public void addWord(String word) {
+        AddAndSearchWordDataStructureDesign pointer = this;
+        char[] chars = word.toCharArray();
+        for (char c : chars) {
+            if (pointer.children[c - 'a'] == null) {
+                AddAndSearchWordDataStructureDesign child =
+                        new AddAndSearchWordDataStructureDesign();
+                child.ch = c;
+                pointer.children[c - 'a'] = child;
+            }
+            pointer = pointer.children[c - 'a'];
+        }
+        pointer.isEnd = true;
+    }
+
+    public boolean search(String word) {
+        AddAndSearchWordDataStructureDesign pointer = this;
+        char[] chars = word.toCharArray();
+        List<Character> charList = new ArrayList<>(chars.length);
+        for (char c : chars) {
+            charList.add(c);
+        }
+        return search(charList, pointer);
+    }
+
+    private boolean search(List<Character> charList, AddAndSearchWordDataStructureDesign pointer) {
+        if (charList.size() == 0) {
+            return false;
+        }
+        if (charList.get(0) == '.') {
+            for (int i = 0; i < 26; i++) {
+                if (pointer.children[i] == null) {
+                    continue;
+                }
+                if (pointer.children[i].isEnd && charList.size() == 1) {
+                    return true;
+                }
+                boolean result = search(charList, pointer.children[i]);
+                if (result) {
+                    return true;
+                }
+                charList.add(0, '.');
+            }
+            return false;
+        } else {
+            if (pointer.children[charList.get(0) - 'a'] == null) {
+                return false;
+            }
+            if (pointer.children[charList.get(0) - 'a'].isEnd && charList.size() == 1) {
+                return true;
+            }
+            Character removed = charList.remove(0);
+            boolean result = search(charList, pointer.children[removed - 'a']);
+            if (result) {
+                return true;
+            }
+            charList.add(0, removed);
+        }
+        return false;
+    }
+}
+
+/**
+ * Your WordDictionary object will be instantiated and called as such: WordDictionary obj = new
+ * WordDictionary(); obj.addWord(word); boolean param_2 = obj.search(word);
+ */
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ImplementTriePrefixTree.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ImplementTriePrefixTree.java
new file mode 100644
index 0000000000000000000000000000000000000000..6c5ac1e26850cd257ef61b347881be1f879ca379
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ImplementTriePrefixTree.java
@@ -0,0 +1,88 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class ImplementTriePrefixTree {
+    private Character val;
+    private boolean isEnd;
+    private Map<Character, ImplementTriePrefixTree> children;
+
+    /** Initialize your data structure here. */
+    public ImplementTriePrefixTree() {
+        val = null;
+        isEnd = false;
+        children = null;
+    }
+
+    /** Inserts a word into the trie. */
+    public void insert(String word) {
+        if (word == null || "".equals(word)) {
+            return;
+        }
+        char[] chars = word.toCharArray();
+        ImplementTriePrefixTree pointer = this;
+        for (char c : chars) {
+            if (pointer.children == null) {
+                pointer.children = new HashMap<>();
+            }
+            if (pointer.children.containsKey(c)) {
+                pointer = pointer.children.get(c);
+            } else {
+                ImplementTriePrefixTree trie = new ImplementTriePrefixTree();
+                trie.val = c;
+                pointer.children.put(c, trie);
+                pointer = trie;
+            }
+        }
+        pointer.isEnd = true;
+    }
+
+    /** Returns if the word is in the trie. */
+    public boolean search(String word) {
+        if (word == null || "".equals(word)) {
+            return false;
+        }
+        char[] chars = word.toCharArray();
+        ImplementTriePrefixTree pointer = this;
+        for (char c : chars) {
+            if (pointer.children == null || pointer.children.isEmpty()) {
+                return false;
+            }
+            ImplementTriePrefixTree child = pointer.children.get(c);
+            if (child == null) {
+                return false;
+            } else {
+                pointer = child;
+            }
+        }
+        return pointer.isEnd;
+    }
+
+    /** Returns if there is any word in the trie that starts with the given prefix. */
+    public boolean startsWith(String prefix) {
+        if (prefix == null || "".equals(prefix)) {
+            return false;
+        }
+        char[] chars = prefix.toCharArray();
+        ImplementTriePrefixTree pointer = this;
+        for (char c : chars) {
+            if (pointer.children == null || pointer.children.isEmpty()) {
+                return false;
+            }
+            ImplementTriePrefixTree child = pointer.children.get(c);
+            if (child == null) {
+                return false;
+            } else {
+                pointer = child;
+            }
+        }
+        return true;
+    }
+}
+
+/**
+ * Your Trie object will be instantiated and called as such: Trie obj = new Trie();
+ * obj.insert(word); boolean param_2 = obj.search(word); boolean param_3 = obj.startsWith(prefix);
+ */
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MapSumPairs.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MapSumPairs.java
new file mode 100644
index 0000000000000000000000000000000000000000..9f611c0a5b892b99be9688f282068a5291d2d5fe
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MapSumPairs.java
@@ -0,0 +1,73 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class MapSumPairs {
+
+    private Character ch;
+    private Map<Character, MapSumPairs> children;
+    private int val;
+
+    /** Initialize your data structure here. */
+    public MapSumPairs() {
+        this.ch = null;
+        this.children = null;
+        this.val = 0;
+    }
+
+    public void insert(String key, int val) {
+        char[] chars = key.toCharArray();
+        MapSumPairs pointer = this;
+        for (char c : chars) {
+            if (pointer.children == null) {
+                pointer.children = new HashMap<>();
+            }
+            if (pointer.children.containsKey(c)) {
+                pointer = pointer.children.get(c);
+            } else {
+                MapSumPairs mapSum = new MapSumPairs();
+                mapSum.ch = c;
+                pointer.children.put(c, mapSum);
+                pointer = mapSum;
+            }
+        }
+        pointer.val = val;
+    }
+
+    public int sum(String prefix) {
+        MapSumPairs pointer = this;
+        char[] chars = prefix.toCharArray();
+        for (char c : chars) {
+            if (pointer.children == null) {
+                return 0;
+            }
+            MapSumPairs child = pointer.children.get(c);
+            if (child == null) {
+                return 0;
+            }
+            pointer = child;
+        }
+        return sum(pointer);
+    }
+
+    private int sum(MapSumPairs mapSum) {
+        if (mapSum == null) {
+            return 0;
+        }
+        if (mapSum.children == null || mapSum.children.isEmpty()) {
+            return mapSum.val;
+        }
+        int sum = 0;
+        for (MapSumPairs child : mapSum.children.values()) {
+            sum += sum(child);
+        }
+        return mapSum.val + sum;
+    }
+}
+
+/**
+ * Your MapSum object will be instantiated and called as such: MapSum obj = new MapSum();
+ * obj.insert(key,val); int param_2 = obj.sum(prefix);
+ */
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MaximumXorOfTwoNumbersInAnArray.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MaximumXorOfTwoNumbersInAnArray.java
new file mode 100644
index 0000000000000000000000000000000000000000..87818ccf5ade69dfde3d6160aded7d1935a874f9
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/MaximumXorOfTwoNumbersInAnArray.java
@@ -0,0 +1,20 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+class MaximumXorOfTwoNumbersInAnArray {
+    public int findMaximumXOR(int[] nums) {
+        if (nums.length == 1) {
+            return 0;
+        }
+        int max = 0;
+        for (int i = 0; i < nums.length - 1; i++) {
+            for (int j = i + 1; j < nums.length; j++) {
+                int xor = nums[i] ^ nums[j];
+                if (xor > max) {
+                    max = xor;
+                }
+            }
+        }
+        return max;
+    }
+}
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ReplaceWords.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ReplaceWords.java
new file mode 100644
index 0000000000000000000000000000000000000000..c1723e179442e1030fe14aa0c593669d1e25f9f0
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/ReplaceWords.java
@@ -0,0 +1,70 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+class ReplaceWords {
+    private class Trie {
+        private Character ch = null;
+        private Map<Character, Trie> children = new HashMap<>();
+        private boolean isEnd = false;
+
+        private void insert(String str) {
+            Trie pointer = this;
+            char[] chars = str.toCharArray();
+            for (char c : chars) {
+                if (pointer.children.containsKey(c)) {
+                    pointer = pointer.children.get(c);
+                } else {
+                    Trie child = new Trie();
+                    child.ch = c;
+                    pointer.children.put(c, child);
+                    pointer = child;
+                }
+            }
+            pointer.isEnd = true;
+        }
+
+        // if no prefix, return str. else return shortest prefix
+        private String prefix(String str) {
+            Trie pointer = this;
+            char[] chars = str.toCharArray();
+            List<Character> result = new ArrayList<>();
+            boolean end = false;
+            for (char c : chars) {
+                if (!pointer.children.containsKey(c)) {
+                    break;
+                } else {
+                    result.add(c);
+                    Trie child = pointer.children.get(c);
+                    if (child.isEnd) {
+                        end = true;
+                        break;
+                    }
+                    pointer = child;
+                }
+            }
+            if (!end) {
+                return str;
+            }
+            return result.stream().map(Object::toString).collect(Collectors.joining(""));
+        }
+    }
+
+    public String replaceWords(List<String> dictionary, String sentence) {
+        Trie trie = new Trie();
+        for (String str : dictionary) {
+            trie.insert(str);
+        }
+        String[] words = sentence.split(" ");
+        List<String> wordList = new ArrayList<>(words.length);
+        for (String word : words) {
+            wordList.add(trie.prefix(word));
+        }
+        return String.join(" ", wordList);
+    }
+}
diff --git a/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/WordSearch2.java b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/WordSearch2.java
new file mode 100644
index 0000000000000000000000000000000000000000..ae56a8e071dcd5d8e5f7ccc746d5234a6dfb0672
--- /dev/null
+++ b/java-leetcode/src/main/java/io/github/poorguy/explore/learn/trie/WordSearch2.java
@@ -0,0 +1,111 @@
+/* All Contributors (C) 2021 */
+package io.github.poorguy.explore.learn.trie;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+/** todo a little slow */
+class WordSearch2 {
+    private class Node {
+        Character ch = null;
+        Node[] children = new Node[26];
+        boolean isEnd = false;
+    }
+
+    private Set<String> result = new HashSet<>();
+
+    public List<String> findWords(char[][] board, String[] words) {
+        Node head = new Node();
+        Node pointer = head;
+        for (String word : words) {
+            for (Character ch : word.toCharArray()) {
+                if (pointer.children[ch - 'a'] == null) {
+                    Node child = new Node();
+                    child.ch = ch;
+                    pointer.children[ch - 'a'] = child;
+                }
+                pointer = pointer.children[ch - 'a'];
+            }
+            pointer.isEnd = true;
+            pointer = head;
+        }
+
+        for (int i = 0; i < board.length; i++) {
+            for (int j = 0; j < board[0].length; j++) {
+                pointer = head;
+                search(
+                        pointer,
+                        i,
+                        j,
+                        board,
+                        new boolean[board.length][board[0].length],
+                        new ArrayList<>());
+            }
+        }
+
+        return new ArrayList<>(result);
+    }
+
+    private boolean search(
+            Node pointer,
+            int row,
+            int col,
+            char[][] board,
+            boolean[][] flag,
+            List<Character> chars) {
+        if (row < 0 || row >= board.length || col < 0 || col >= board[0].length || flag[row][col]) {
+            return false;
+        }
+
+        char ch = board[row][col];
+        Node child = pointer.children[ch - 'a'];
+        if (child == null) {
+            return false;
+        }
+        flag[row][col] = true;
+
+        chars.add(child.ch);
+        if (child.isEnd) {
+            result.add(listToString(chars));
+        }
+
+        boolean hasNext = false;
+        if (search(child, row - 1, col, board, flag, chars)) {
+            hasNext = true;
+        }
+        if (search(child, row + 1, col, board, flag, chars)) {
+            hasNext = true;
+        }
+        if (search(child, row, col - 1, board, flag, chars)) {
+            hasNext = true;
+        }
+        if (search(child, row, col + 1, board, flag, chars)) {
+            hasNext = true;
+        }
+        chars.remove(chars.size() - 1);
+        flag[row][col] = false;
+        return child.isEnd || hasNext;
+    }
+
+    private String listToString(List<Character> list) {
+        StringBuilder sb = new StringBuilder();
+        for (Character ch : list) {
+            sb.append(sb);
+        }
+        return sb.toString();
+    }
+
+    // [["o","a","a","n"],["e","t","a","e"],["i","h","k","r"],["i","f","l","v"]]
+    // ["oath","pea","eat","rain"]
+    public static void main(String[] args) {
+        WordSearch2 solution = new WordSearch2();
+        char[][] board = {
+            {'o', 'a', 'a', 'n'}, {'e', 't', 'a', 'e'}, {'i', 'h', 'k', 'r'}, {'i', 'f', 'l', 'v'}
+        };
+        String[] words = {"oath", "pea", "eat", "rain"};
+        List<String> wordList = solution.findWords(board, words);
+        wordList.forEach(System.out::println);
+    }
+}