20. 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.
检测括号是否有效，这个是使用栈的超级合适的地方。
入栈的时候如果是反括号就检查栈顶是不是对应的正括号，不是的话就直接返回错就好啦，最后检测一下栈是不是空的。

var isValid = function(s) {
    var num = s.length;
    if (num===0)
        return true;
    var stack = [];
    var map = {
        ')':'(',
        ']':'[',
        '}':'{',
    };
    for (var i = 0;i<num;i++) {
        if (stack.length!==0) {
            if (map[s[i]]!==undefined) {
                if (stack[stack.length-1]!==map[s[i]])
                    return false;
                else
                    stack.pop();
            } else 
                stack.push(s[i]);
        } else
            stack.push(s[i]);
    }
    if (stack.length!==0)
        return false;
    return true;
};