Valid Parentheses

import java.util.Stack;

public class JudgeExact {

    public static boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return false;
        }
        char[] cset = s.toCharArray();
        Stack<Character> characters = new Stack<Character>();
        int i = 0;
        int len = cset.length;
        if (len == 1) {
            return false;
        }
        while (i < len) {
            if (characters.isEmpty()) {
                characters.add(cset[i]);
            } else {
                char left = characters.peek();
                char right = cset[i];
                if ((left == '{' && right == '}')
                        || (left == '(' && right == ')')
                        || (left == '[' && right == ']')) {
                    characters.pop();
                } else {
                    characters.push(cset[i]);
                }

            }
            i++;
        }
        if (characters.isEmpty()) {
            return true;
        }
        return false;

    }
    public static void main(String[] args) {
        System.out.println(JudgeExact.isValid("{"));

    }
}

 

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.