括号匹配

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.

public static boolean isValid(String s) {
        if (s == null || s.length() <= 1) return false;
        Stack<Character> bracket = new Stack<Character>();
        bracket.push(s.charAt(0));
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') {
                bracket.push(s.charAt(i));
            } else if (s.charAt(i) == ')' && bracket.size() !=0 && bracket.peek() == '(') {
                    bracket.pop();
            } else if (s.charAt(i) == ']' && bracket.size() !=0 && bracket.peek() == '[') {
                    bracket.pop();
            } else if (s.charAt(i) == '}' && bracket.size() !=0 && bracket.peek() == '{') {
                    bracket.pop();
            } else {
                return false;
            }
        }
        if (bracket.size() != 0) return false;
           return true;
    }