20190305

code/lc344.java

+package code;
+/*
+ * 344. Reverse String
+ * 题意：反转字符串
+ * 难度：Easy
+ * 分类：Two Pointers, String
+ * 思路：
+ * Tips：
+ */
+public class lc344 {
+    public void reverseString(char[] s) {
+        int begin = 0, end = s.length-1;
+        while(begin<end){
+            char ch = s[begin];
+            s[begin] = s[end];
+            s[end] = ch;
+            begin++;
+            end--;
+        }
+    }
+}

code/lc378.java

 package code;
+
+import java.util.Comparator;
+import java.util.PriorityQueue;
+
 /*
  * 378. Kth Smallest Element in a Sorted Matrix
  * 题意：在矩阵中搜索第k大的数，横轴和纵轴都是有序的
  * 难度：Medium
  * 分类：Binary Search, Heap
- * 思路：
+ * 思路：两种思路。 1是类似多个有序链表合并的思路，优先队列。
+ *                2是二分，二分的是val，看比这个val小的数是不是k
  * Tips：lc23方法很像
  */
 public class lc378 {
+    class Cell{
+        int val, row, col;
+        Cell(int v, int r, int c){
+            val = v;
+            row = r;
+            col = c;
+        }
+    }
     public int kthSmallest(int[][] matrix, int k) {
-        
+        PriorityQueue<Cell> pq = new PriorityQueue(new Comparator<Cell>() {
+            @Override
+            public int compare(Cell o1, Cell o2) {
+                return o1.val-o2.val;
+            }
+        });
+        for (int i = 0; i < matrix.length ; i++) pq.add(new Cell(matrix[i][0], i, 0));
+        while(k>1){
+            Cell c = pq.remove();
+            if(c.col+1<matrix[0].length) pq.add(new Cell(matrix[c.row][c.col+1], c.row, c.col+1));
+            k--;
+        }
+        return pq.remove().val;
+    }
+
+    public int kthSmallest2(int[][] matrix, int k) {
+        int low = matrix[0][0];
+        int high = matrix[matrix.length-1][matrix[0].length-1];
+        while(low<=high){   //确保区间最后缩到0
+            int mid = low+(high-low)/2;
+            int num = getLessNum(matrix, mid);
+            if(num<k) low = mid+1;
+            else high = mid-1;
+        }
+        return low-1;
+    }
+    public int getLessNum(int[][] matrix, int val){ //求矩阵中比这个数小的数的个数
+        int res = 0;
+        int row = 0;
+        while(row<matrix.length){
+            int col = 0;
+            while(col<matrix[0].length && matrix[row][col]<val) col++;
+            res += col;
+            row++;
+        }
+        return res;
     }
 }

code/lc380.java

+package code;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+/*
+ * 380. Insert Delete GetRandom O(1)
+ * 题意：设计一个数据结构，插入，删除，随机获得一个元素 这三个操作的复杂度都为O(1)
+ * 难度：Medium
+ * 分类：Array, Hash Table, Design
+ * 思路：List 的插入和删除都是O(1), 通过hashmap绑定来使得Get也为O(1)
+ * Tips：
+ */
+public class lc380 {
+    public class RandomizedSet {
+
+        HashMap<Integer, Integer> valToInd;
+        List<Integer> list;
+        int ind = 0;
+
+        /** Initialize your data structure here. */
+        public RandomizedSet() {
+            valToInd = new HashMap<>();
+            list = new ArrayList<>();
+        }
+
+        /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
+        public boolean insert(int val) {
+            if(valToInd.containsKey(val)) return false;
+            list.add(val);
+            valToInd.put(val,list.size()-1);
+            return true;
+        }
+
+        /** Removes a value from the set. Returns true if the set contained the specified element. */
+        public boolean remove(int val) {
+            int ind = valToInd.getOrDefault(val,-1);
+            if(ind == -1) return false;
+            Collections.swap(list,ind,list.size()-1);
+            int swappedWith = list.get(ind);
+            valToInd.put(swappedWith,ind);
+            list.remove(list.size()-1);
+            valToInd.remove(val);
+            return true;
+        }
+
+        /** Get a random element from the set. */
+        public int getRandom() {
+            int max = list.size();
+            int min = 0;
+            int ind = (int)(Math.random() * (max - min) + min);
+            return list.get(ind);
+        }
+    }
+}

code/lc384.java

+package code;
+/*
+ * 384. Shuffle an Array
+ * 题意：重排列一个数组，每个值在每个位置的概率都是均匀的
+ * 难度：Medium
+ * 分类：
+ * 思路：
+ * Tips：
+ */
+public class lc384 {
+    public class Solution {
+
+        private int[] nums;
+
+        public Solution(int[] nums) {
+            this.nums = nums;
+        }
+
+        /** Resets the array to its original configuration and return it. */
+        public int[] reset() {
+            return nums;
+        }
+
+        /** Returns a random shuffling of the array. */
+        public int[] shuffle() {
+            int[] rand = new int[nums.length];
+            for (int i = 0; i < nums.length; i++){
+                int r = (int) (Math.random() * (i+1));
+                rand[i] = rand[r];
+                rand[r] = nums[i];
+            }
+            return rand;
+        }
+    }
+}

code/lc454.java

+package code;
+
+import java.util.HashMap;
+
+/*
+ * 454. 4Sum II
+ * 题意：从4个数组中各挑一个数，使得和为0
+ * 难度：Medium
+ * 分类：Hash Table, Binary Search
+ * 思路：自己没想起来，看了答案后感觉很无聊
+ *      两个集合暴力求出所有和的可能，然后2Sum的思路，利用Hashmap即可
+ *      其实是利用了二分的思想，把4个数的和变为两个2个数的和
+ * Tips：
+ */
+public class lc454 {
+    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
+        HashMap<Integer, Integer> hm = new HashMap();
+        int res=0;
+        for(int i=0; i<A.length; i++){
+            for(int j=0; j<B.length; j++){  //从0开始
+                hm.put(A[i]+B[j],hm.getOrDefault(A[i]+B[j],0)+1);
+            }
+        }
+        for(int i=0; i<C.length; i++){
+            for(int j=0; j<D.length; j++){
+                res+=hm.getOrDefault(-C[i]-D[j],0);
+            }
+        }
+        return res;
+    }
+}

code/lc76.java

@@ -6,6 +6,7 @@ package code;
  * 分类：Hash Table, Two Pointers, String
  * 思路：两个指针，移动右指针使得满足条件，移动左指针缩短距离。用hashmap存储进行判断是否满足条件。
  * Tips：很难的题，思路记一下。
+ *      https://leetcode.com/problems/minimum-window-substring/discuss/26808/here-is-a-10-line-template-that-can-solve-most-substring-problems
  */
 import java.util.HashMap;