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.

括号匹配，使用栈求解。

class Solution {
public:
    bool isValid(string s) {
        stack<char>a;
        int n = s.size();
        for (int i = 0;i < n;i++) {
            if (s[i] == '(' || s[i] == '{' || s[i] == '[') {
                a.push(s[i]);
            }
            else if (s[i] == ')' || s[i] == '}' || s[i] == ']') {
                if (a.empty()) return false;
                if (s[i] == ')' && a.top() == '(') { a.pop();  continue; }
                if (s[i] == '}' && a.top() == '{') { a.pop();  continue; }
                if (s[i] == ']' && a.top() == '[') { a.pop();  continue; }
                return false;
            }
        }
        if (a.empty()) return true;
        else return false;
    }
};