LeetCode 020 Valid Parentheses

LeetCode

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Example 1:

Input: s = “()”
Output: true

Example 2:

Input: s = “()[]{}”
Output: true

Example 3:

Input: s = “(]”
Output: false

Example 4:

Input: s = “([)]”
Output: false

Example 5:

Input: s = “{[]}”
Output: true

Constraints:

1 <= s.length <= 10<sup>4</sup> s consists of parentheses only '()[]{}'.

题目大意：

求给定字符串是否合法括号配对。

算法思路：

括号题优先考虑用Stack

注意事项：

三种不合法情况： ‘[‘ (stack有余), ‘]’ (要匹配的时候stack为空), ‘{]’ (不匹配)

Python代码：

PARENTHESES_DICT = {'(': ')', '[': ']', '{': '}'}
class Solution:
    def isValid(self, s: str) -> bool:
        if not s:
            return False
        stack = []
        for char in s:
            if char in '([{':
                stack.append(char)
            else:
                if not stack:
                    return False
                left = stack.pop()
                if PARENTHESES_DICT[left] != char:
                    return False
        return True if not stack else False

算法分析：

时间复杂度为O(n)，空间复杂度O(n)