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.

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length()==0) {
            return true;
        }
        if (s.length() == 1) {
            return false;
        }
        Stack<Character> stack =  new Stack<>();//use stack 
        char[] sch = s.toCharArray();
        int index = 0;
        while (index < sch.length) {
            if (sch[index] == '(' || sch[index] == '[' ||sch[index] == '{') {
                stack.push(sch[index]);
                index++;
                continue;
            }
            if (sch[index] == ')') {
                if(stack.isEmpty() || stack.pop() != '(' ) {
                    return false;
                }

            }else if (sch[index] == ']') {
                 if(stack.isEmpty() ||stack.pop() != '[' ) {
                    return false;
                }
            }else if (sch[index] == '}') {
                if(stack.isEmpty() ||stack.pop() != '{' ) {
                    return false;
                }
            }
            index++;
        }
        return stack.isEmpty();
    }
}

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }
        if (s.length() == 1) {
            return false;
        }
        Stack<Character> stack = new Stack<>();
        char[] sch = s.toCharArray();
        for (char c : sch) {
            if (c == '(') {
                stack.push(')');
            }else if(c == '[') {
                stack.push(']');
            }else if (c == '{') {
                stack.push('}');
            }else{
                if (stack.isEmpty() || stack.pop() != c) {
                    return false;
                } 
            }

        }
        return stack.isEmpty();
    }
}