Leetcode: 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.

分析：一个很简单的想法是用栈保存左括号。时间复杂度O(n), 空间复杂度O(n)。

class Solution {
public:
    bool isValid(string s) {
        int n = s.length();
        if(n == 0) return true;
        
        stack<char> S;
        for(int i = 0; i < n; i++){
            if(s[i] == '(' || s[i] == '{' || s[i] == '[')
                S.push(s[i]);
            else if(!S.empty() && match(S.top(), s[i]))
                    S.pop();
            else return false;
        }
        
        return S.empty();
    }
    
    bool match(char l, char r){
        return l == '(' && r == ')' || l == '{' && r == '}' || l == '[' && r == ']';
    }
};