20. Valid Parentheses

Given a string 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.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

思路

应该说非常简单了，第一时间想到的就是栈：

• 如果为空，则入栈
• 碰到右括号，判断栈顶是否是其对应的左括号，如果是，则出栈，不是，则继续入栈
• 最后如果栈为空，表示括号对一一匹配，返回true，否则为false

代码

class Solution {
public:
    bool isValid(string s) {
        stack<char> stk;
        for (auto c : s)
            if (!stk.empty() && (\
                (c == ')' && stk.top() == '(') || \
                (c == ']' && stk.top() == '[') || \
                (c == '}' && stk.top() == '{')))
                stk.pop();
            else stk.push(c);
        return stk.empty();
    }
};