Valid Parentheses

【Leetcode 20】
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 boolean isValid(String s) {
        // check corner case
        if (s == null || s.length() == 0 || s.length() % 2 != 0) return false;

        Stack<Character> stack = new Stack<>();

        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == '[' || ch == '(' || ch == '{') {
                stack.push(ch);
            } else if (ch == ']' && !stack.isEmpty() && stack.peek() == '[') {
                stack.pop();
            } else if (ch == ')' && !stack.isEmpty() && stack.peek() == '(') {
                stack.pop();
            } else if (ch == '}' && !stack.isEmpty() && stack.peek() == '{') {
                stack.pop();
            } else {
                // if all above are not satisfied, then it's a illegal operation
                return false;
            }
        }
        // stack should be empty at last
        return stack.isEmpty();
    }