From 9b3853eae2761011f82b77aa45b840482e1b472b Mon Sep 17 00:00:00 2001
From: Baturu <45113401+z80160280@users.noreply.github.com>
Date: Tue, 8 Jun 2021 21:05:11 -0700
Subject: [PATCH] =?UTF-8?q?Update=200647.=E5=9B=9E=E6=96=87=E5=AD=90?=
 =?UTF-8?q?=E4=B8=B2.md?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 problems/0647.回文子串.md | 50 +++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/problems/0647.回文子串.md b/problems/0647.回文子串.md
index 58949571..31734bbc 100644
--- a/problems/0647.回文子串.md
+++ b/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