feat: longest-palindromic-substring

Author: junbinding

str.longest-palindromic-substring.py

@@ -0,0 +1,41 @@
+class Solution:
+    """
+    5. 最长回文子串
+    https://leetcode-cn.com/problems/longest-palindromic-substring/
+    给定一个字符串 s，找到 s 中最长的回文子串。你可以假设 s 的最大长度为 1000。
+    """
+    def longestPalindrome(self, s: str) -> str:
+        l = len(s)
+        if l < 2:
+            return s
+
+        # 定义状态方程，dp[i][j] 表示 i 到 j 是否是回文串
+        dp = [[False] * l for _ in range(l)]
+
+        max_len = 1
+        start = 0
+
+        # i 到 i 必然是 True
+        for i in range(l):
+            dp[i][i] = True
+
+        for j in range(1, l):
+            for i in range(0, j):
+                # 状态转移方程
+                if s[i] == s[j]:
+                    if j - i < 3:
+                        dp[i][j] = True
+                    else:
+                        dp[i][j] = dp[i+1][j-1]
+                else:
+                    dp[i][j] = False
+                # 记录开始位置，以及最大长度
+                if dp[i][j] and (j - i + 1) > max_len:
+                    max_len = j - i + 1
+                    start = i
+
+        return s[start: start + max_len]
+
+
+so = Solution()
+print(so.longestPalindrome('cbbd'))