1.题目

www.nowcoder.com/practice/b4…

2. 考点

1. 双指针
2. 中心扩散
3. 回文串 例如:noon aba等 A[l:r] == A[l:r][::-1]

3. 解析

使用双指针

4. 核心代码

双指针解法
class Solution:
    def getLongestPalindrome(self , A: str) -> int:
        n = len(A)
        if n == 1: return 1
        l = 0
        r = 1
        ret = 0
        while l <= r <= n:
            while r <= n:
                if A[l:r] == A[l:r][::-1]:
                    ret = max(ret, r - l)
                r += 1
            l += 1
            r = l + 1
        return ret

中心扩散法
class Solution:
    def fun(self, s: str, begin: int, end: int) -> int:
        #每个中心点开始扩展
        while begin >= 0 and end < len(s) and s[begin] == s[end]:
            begin -= 1
            end += 1
        #返回长度
        return end - begin - 1
       
    def getLongestPalindrome(self , A: str) -> int:
        maxlen = 1
        #以每个点为中心
        for i in range(len(A) - 1):
            #分奇数长度和偶数长度向两边扩展
            maxlen = max(maxlen, max(self.fun(A, i, i), self.fun(A, i, i + 1)))
        return maxlen

4. 代码优化

# 如需要返回长度 只需要返回len就可以了
class Solution:
    def longestPalindrome(self, s: str) -> str:
        if not s:
            return ""
        if s[::-1] == s:
            return s
        n = len(s)
        left = 1
        temps = s[0]
        for i in range(n):
            if i - left >= 1 and s[i - left - 1:i + 1] == s[i - left - 1:i + 1][::-1]:
                temps = s[i - left - 1:i + 1]
                left += 2
            elif i - left >= 0 and s[i - left:i + 1] == s[i - left:i + 1][::-1]:
                temps = s[i - left:i + 1]
                left += 1
        return temps

5. 很好理解法

采用中心扩散 如果为奇数就为 _is_palindrome(s, i, i) 如果是偶数_is_palindrome(s, i, i + 1) 最后获取最长的那个结果

class Solution:
    def longestPalindrome(self, s: str) -> str:
        def _is_palindrome(ss, l, r):
            while l >= 0 and r < len(ss) and ss[l] == ss[r]:
                l -= 1
                r += 1
            return ss[l + 1: r]

        res = ""
        for i in range(len(s)):
            s1 = _is_palindrome(s, i, i)
            s2 = _is_palindrome(s, i, i + 1)
            res = s1 if len(s1) > len(res) else res
            res = s2 if len(s2) > len(res) else res
        return res