字符串

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        # 哈希集合，记录每个字符是否出现过
        occ = set()
        n = len(s)
        # 右指针，初始值为 -1，相当于我们在字符串的左边界的左侧，还没有开始移动
        rk, ans = -1, 0
        for i in range(n):
            if i != 0:
                # 左指针向右移动一格，移除一个字符
                occ.remove(s[i - 1])
            while rk + 1 < n and s[rk + 1] not in occ:
                # 不断地移动右指针
                occ.add(s[rk + 1])
                rk += 1
            # 第 i 到 rk 个字符是一个极长的无重复字符子串
            ans = max(ans, rk - i + 1)
        return ans

class Solution:
    def isValid(self, s: str) -> bool:
        if len(s) % 2 == 1:
            return False
        
        pairs = {
            ")": "(",
            "]": "[",
            "}": "{",
        }
        stack = list()
        for ch in s:
            if ch in pairs:
                if not stack or stack[-1] != pairs[ch]:
                    return False
                stack.pop()
            else:
                stack.append(ch)
        
        return not stack

class Solution:
    def expandAroundCenter(self, s, left, right):
        while left >= 0 and right < len(s) and s[left] == s[right]:
            left -= 1
            right += 1
        return left + 1, right - 1

    def longestPalindrome(self, s: str) -> str:
        start, end = 0, 0
        for i in range(len(s)):
            left1, right1 = self.expandAroundCenter(s, i, i)
            left2, right2 = self.expandAroundCenter(s, i, i + 1)
            if right1 - left1 > end - start:
                start, end = left1, right1
            if right2 - left2 > end - start:
                start, end = left2, right2
        return s[start: end + 1]