LeetCode题解(0020)：有效的括号(Python)

题目：题目链接（简单）

标签：栈、字符串

解法	时间复杂度	空间复杂度	执行用时
Ans 1 (Python)	$O(N)$	$O(N)$	36ms (89.92%)
Ans 2 (Python)	$O(N)$	$O(N)$	36ms (89.92%)
Ans 3 (Python)	$O(N^2)$	$O(N)$	52ms (20.39%)

解法一（使用列表存储当前括号状态）：

def isValid(self, s: str) -> bool:

    sign_dict = {
        ")": "(",
        "]": "[",
        "}": "{"
    }

    sign_list = []
    for c in s:
        if c in ["(", "[", "{"]:
            sign_list.append(c)
        else:
            r = sign_dict[c]
            if sign_list and r == sign_list[-1]:
                sign_list.pop()
            else:
                return False

    if sign_list:
        return False
    else:
        return True

解法二：

def isValid(self, s: str) -> bool:
    stack = []
    for c in s:
        if c == "(" or c == "[" or c == "{":
            stack.append(c)
        else:
            if not stack:
                return False
            elif c == ")" and stack[-1] != "(":
                return False
            elif c == "]" and stack[-1] != "[":
                return False
            elif c == "}" and stack[-1] != "{":
                return False
            stack.pop()
    return not stack

解法三（字符串替换）：

def isValid(self, s: str) -> bool:
    last = len(s)
    new = 0
    while last != new:
        last = len(s)
        s = s.replace("()", "")
        s = s.replace("[]", "")
        s = s.replace("{}", "")
        new = len(s)
    return new == 0