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"
思路
看到官方的Solution才明白过来这是一个动态规划的问题。
如果一个字符串是回文字符串，那么在它两侧同时加上相同的字符，依然是回文字符串。
即s[l]==s[r]，要考虑到回文字符串本身长度是奇数还是偶数的问题。
class Solution:
def longestPalindrome(self, s):
"""
:type s: str
:rtype: str
"""
if len(s) < 2:
return s
res = ""
for i in range(len(s)):
# 当回文字符串长度为奇数时
tmp = self.helper(s, i, i)
if len(tmp) > len(res):
res = tmp
# 当回文字符串长度为偶数时
tmp = self.helper(s, i, i + 1)
if len(tmp) > len(res):
res = tmp
return res
def helper(self, s, l, r):
while l >= 0 and r < len(s) and s[l] == s[r]:
l -= 1
r += 1
return s[l+1 : r]
if __name__ == '__main__':
solution = Solution()
s = "cbbd"
res = solution.longestPalindrome(s)
print(res)
