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new file mode 100644
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+++ b/.idea/awesome-algorithm.iml
@@ -0,0 +1,9 @@
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new file mode 100644
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--- /dev/null
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@@ -0,0 +1,4 @@
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new file mode 100644
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@@ -0,0 +1,36 @@
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new file mode 100644
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+++ b/.idea/misc.xml
@@ -0,0 +1,7 @@
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diff --git a/.idea/modules.xml b/.idea/modules.xml
new file mode 100644
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--- /dev/null
+++ b/.idea/modules.xml
@@ -0,0 +1,8 @@
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\ No newline at end of file
diff --git a/.idea/vcs.xml b/.idea/vcs.xml
new file mode 100644
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--- /dev/null
+++ b/.idea/vcs.xml
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new file mode 100644
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--- /dev/null
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\ No newline at end of file
diff --git a/docs/Leetcode_Solutions/Java/0004._Median_of_Two_Sorted_Arrays.md b/docs/Leetcode_Solutions/Java/0004._Median_of_Two_Sorted_Arrays.md
new file mode 100644
index 0000000000000000000000000000000000000000..f540dafe322bdd620ded84a76a143b48c3a434e2
--- /dev/null
+++ b/docs/Leetcode_Solutions/Java/0004._Median_of_Two_Sorted_Arrays.md
@@ -0,0 +1,78 @@
+#  4. Longest Palindromic Substring
+
+**<font color=red>难度: Medium</font>**
+
+## 刷题内容
+
+> 原题连接
+
+* https://leetcode-cn.com/problems/longest-palindromic-substring/description
+
+> 内容描述
+
+```
+Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
+
+
+Example 1:
+
+Input: "babad"
+Output: "bab"
+Note: "aba" is also a valid answer.
+
+
+Example 2:
+
+Input: "cbbd"
+Output: "bb"
+
+```
+
+## 解题方案
+
+> 思路 1
+******- 时间复杂度: O(n^2)******- 空间复杂度: O(n^2)******
+
+使用动态规划的思路,用一个二维数组cache[i][j]记录i到j是否为回文串, beats 54.36%
+
+```java
+class Solution {
+    // 采用动态规划
+    // 如果 i == j ，则只有一个字母 肯定是回文串
+    // 如果 char[i] == char[j] && j == i + 1 , 两个字母相等 肯定是回文串
+    // 如果 char[i] == char[j] && j > i + 1 && cache[i+1][j-1]为true，则肯定是回文串
+    public String longestPalindrome(String s) {
+        if(s == null || s.length() <2){
+            return s;
+        }
+        boolean[][] cache = new boolean[s.length()][s.length()]; // 记录 i ~ j 是否是回文串
+        char[] chars = s.toCharArray();
+        int len = s.length();
+        int start = 0;
+        int end = 0;
+        // 采用至底向上的动态规划，也可以采用递归方式
+        for(int i = len - 1; i >= 0; i --){
+            for(int j = i; j < len; j ++){
+                if(i == j){
+                    cache[i][j] = true;    
+                }else if(j == i + 1 && chars[i] == chars[j]){
+                    cache[i][j] = true;
+                    if(end - start + 1 < 2){
+                        end = j;
+                        start = i;
+                    }
+                }else if(chars[i] == chars[j] && cache[i + 1][j-1]){
+                    cache[i][j] = true;
+                    if(end - start  < j-i){
+                        start = i;
+                        end = j;
+                    }
+                }else{
+                    cache[i][j] = false;
+                }
+            }
+        }
+        return s.substring(start,end+1);
+    }
+}
+```
diff --git a/docs/Leetcode_Solutions/Java/0005._Longest_Palindromic_Substring b/docs/Leetcode_Solutions/Java/0005._Longest_Palindromic_Substring
new file mode 100644
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