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.

Tips:

用stack， 看到左括号就入栈，看到右括号就出栈看能不能匹配到左括号。

Code：

public class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }
        Stack<Character> stack = new Stack<>();

        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.push('(');
                continue;
            }
            if (s.charAt(i) == '{') {
                stack.push('{');
                continue;
            }            
            if (s.charAt(i) == '[') {
                stack.push('[');
                continue;
            }
            if (s.charAt(i) == ']') {
                if (stack.isEmpty()) {
                    return false;
                }
                if (stack.pop() != '[') {
                    return false;
                }
            }
            if (s.charAt(i) == '}') {
                if (stack.isEmpty()) {
                    return false;
                }
                if (stack.pop() != '{') {
                    return false;
                }
            }
            if (s.charAt(i) == ')') {
                if (stack.isEmpty()) {
                    return false;
                }
                if (stack.pop() != '(') {
                    return false;
                }
            }            
        }

        if (stack.isEmpty()) {
            return true;
        }
        return false;
    }
}