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.

Use a stack, if we see '{', '[', '(' the push to stack, if we see '}', ']', ')', check if it matches the top element in the stack, if match, pop.

When matching '}', ']', ')', stack shouldn't be empty, like "{}]".

public boolean isValid(String s) {
        if (s==null || s.length()<2) {
            return false;
        }

        Stack<Character> stack = new Stack<Character>();
        Map<Character, Character> parentheses = new HashMap<Character, Character>();

        parentheses.put('{', '}');
        parentheses.put('[', ']');
        parentheses.put('(', ')');

        for (int i=0; i<s.length(); i++) {
            char c = s.charAt(i);
            if (parentheses.containsKey(c)) {
                stack.push(c);
            } else if (parentheses.containsValue(c)){
                //right parenthese has not yet been pushed to stack [])
                if (!stack.isEmpty() && parentheses.get(stack.peek()) == c) {
                    stack.pop();
                } else {
                    return false;
                }
            }
        }    
        return stack.isEmpty();
    }