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.

给一串只包含有'(', ')', '{', '}', '[' 和']',的字符串，要求判定是否是合法的。

    public boolean isValid(String s) {
        Stack<Character> seq = new Stack<>();
        for (Character c:s.toCharArray())
        {
            if (c=='(') seq.push(')');
            else if (c=='[') seq.push(']');
            else if (c=='{') seq.push('}');
            else if (seq.isEmpty() || seq.pop()!= c) return false;
        }
        return seq.isEmpty();        
    }