添加 0005.最长回文子串.md C语言双指针解法

problems/0005.最长回文子串.md

@@ -513,5 +513,41 @@ char * longestPalindrome(char * s){
 }
 ```
 
+双指针:
+```c
+int left, maxLength;
+void extend(char *str, int i, int j, int size) {
+    while(i >= 0 && j < size && str[i] == str[j]) {
+        //若当前子字符串长度大于最长的字符串长度，进行更新
+        if(j - i + 1 > maxLength) {
+            maxLength = j - i + 1;
+            left = i;
+        }
+        //左指针左移，右指针右移。扩大搜索范围
+        ++j, --i;
+    }
+}
+
+char * longestPalindrome(char * s){
+    left = right = maxLength = 0;
+    int size = strlen(s);
+
+    int i;
+    for(i = 0; i < size; ++i) {
+        //长度为单数的子字符串
+        extend(s, i, i, size);
+        //长度为双数的子字符串
+        extend(s, i, i + 1, size);
+    }
+
+    //复制子字符串
+    char *subStr = (char *)malloc(sizeof(char) * (maxLength + 1));
+    memcpy(subStr, s + left, maxLength);
+    subStr[maxLength] = 0;
+
+    return subStr;
+}
+```
+
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