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. 

可以使用Stack,扫描一遍String即可，当前字符为’（’，'[','{' 压入栈中，当遇到的字符为')',']','}'时弹出栈顶元素判断是否与其匹配，如果不匹配或stack为空返回false.

当扫描完String后如果Stack的元素非空返回false,否则返回true.

public class Solution {
    public boolean isValidParentheses(String s) {
        Stack<Character> stack = new Stack<Character>();
        for (Character c : s.toCharArray()) {
        if ("({[".contains(String.valueOf(c))) {
                stack.push(c);
            } else {
               if (!stack.isEmpty() && is_valid(stack.peek(), c)) {
                   stack.pop();
               } else {
                   return false;
               }
           }
       }
       return stack.isEmpty();
    }

    private boolean is_valid(char c1, char c2) {
        return (c1 == '(' && c2 == ')') || (c1 == '{' && c2 == '}')
            || (c1 == '[' && c2 == ']');
    }
}