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.

public class Solution {
    public boolean isValid(String s) {
            if (s == null || s.length() % 2 == 1) {
                return false;
            }

            Map<Character, Character> parenMap = new HashMap<Character, Character>();
            parenMap.put(')', '(');
            parenMap.put(']', '[');
            parenMap.put('}', '{');

            char [] parenChars = s.toCharArray();
            Stack<Character> currentParen = new Stack<Character>();
            for (char parenChar : parenChars) {
                Character targetValue = parenMap.get(parenChar);
                if (targetValue == null) {
                    currentParen.push(parenChar);
                } else {
                    if (currentParen.isEmpty()) {
                        return false;
                    }

                    if (! currentParen.pop().equals(targetValue)){
                        return false;
                    }
                }
            }

            if (currentParen.isEmpty()) {
                return true;
            }

            return false;          
    }
}

public class Solution {
    public boolean isValid(String s) {
        Map<Character, Character> map = new HashMap<>();
        map.put(')', '(');
        map.put(']','[');
        map.put('}','{');

        Stack<Character> stack = new Stack<>();
        for(char ch : s.toCharArray()){
            if(ch == '(' || ch == '[' || ch == '{'){
                stack.push(ch);
            }else{
                char pair = map.get(ch);
                if(stack.isEmpty() || stack.peek() != pair) return false;
                else stack.pop();
            }
        }

        return stack.isEmpty();
    }
}