LeetCode 20: Valid Parentheses

Valid Parentheses

Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[’ and ‘]’, determine if the input string is valid. The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

解题思路

使用栈（stack）即可解决该问题：代码如下：

class Solution {
public:
    bool isValid(string s) {
        stack<char> cs;
        int slen = s.length();
        for (int i = 0; i < slen; ++i) {
            if (s[i] == '(' || s[i] == '{' || s[i] == '[') {
                // 左括号入栈
                cs.push(s[i]);
            }
            else {
                // 右括号必须和栈顶的左括号匹配
                // 栈空不匹配
                if (cs.empty()) return false;

                // 分情况判断是否匹配
                switch (cs.top()) {
                    case '(': 
                        if (s[i] != ')') return false;
                        break;
                    case '{': 
                        if (s[i] != '}') return false;
                        break;
                    case '[': 
                        if (s[i] != ']') return false;
                        break;
                    defalut: return false;
                }
                cs.pop();
            }
        }
        return cs.empty(); // 栈是否为空？
    }
};