Update 0647.回文子串.md

problems/0647.回文子串.md

@@ -284,6 +284,56 @@ class Solution {
 
 Python：
 
+> 动态规划：
+```python
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        dp = [[False] * len(s) for _ in range(len(s))]
+        result = 0
+        for i in range(len(s)-1, -1, -1): #注意遍历顺序
+            for j in range(i, len(s)):
+                if s[i] == s[j]:
+                    if j - i <= 1: #情况一 和 情况二
+                        result += 1
+                        dp[i][j] = True
+                    elif dp[i+1][j-1]: #情况三
+                        result += 1
+                        dp[i][j] = True
+        return result
+```
+
+> 动态规划：简洁版
+```python
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        dp = [[False] * len(s) for _ in range(len(s))]
+        result = 0
+        for i in range(len(s)-1, -1, -1): #注意遍历顺序
+            for j in range(i, len(s)):
+                if s[i] == s[j] and (j - i <= 1 or dp[i+1][j-1]): 
+                    result += 1
+                    dp[i][j] = True
+        return result
+```
+
+> 双指针法：
+```python
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        result = 0
+        for i in range(len(s)):
+            result += self.extend(s, i, i, len(s)) #以i为中心
+            result += self.extend(s, i, i+1, len(s)) #以i和i+1为中心
+        return result
+
+    def extend(self, s, i, j, n):
+        res = 0
+        while i >= 0 and j < n and s[i] == s[j]:
+            i -= 1
+            j += 1
+            res += 1
+        return res
+```
 
 Go：
 ```Go