Valid Parentheses @leetcode

ValidParentheses.java

package leetcode;

import java.util.Stack;

/**
 * Solution: Stack left --> push, right -->pop and compare
 * Didn't check if s is valid. like "", a little vague here, need to ask interviewer.
 * 
 * The problem says this string contains just these 6 char but not other! read carefully!
 * the last part check is stack.size() > 0. use return stack.isEmpty() to refactor. 
 * 
 * @author jeffwan
 * @date Apr 2, 2014
 */
public class ValidParentheses {
	public boolean isValid(String s) {
		Stack<Character> stack = new Stack<Character>();
		for (Character c : s.toCharArray()) {
			if ("{([".contains(String.valueOf(c))) {
				stack.push(c);
			} else {
				if (!stack.isEmpty() && isValidHelper(stack.peek(), c)){
					stack.pop();
				} else {
					return false;
				}
			}
		}
		
		return stack.isEmpty();
	}
	
	private boolean isValidHelper(char c1, char c2) {
		return (c1 == '(' && c2 == ')') || (c1 == '{' && c2 == '}')
				|| (c1 == '[' && c2 == ']');
	}
	
	// My solution -- thought is clear but code is not very clear.
	public static boolean isValid2(String s) {
		Stack<Character> stack = new Stack<Character>();
		
		for (int i = 0; i < s.length(); i++) {
			char c = s.charAt(i);
			if (c == '(' || c == '{' || c == '[') {
				stack.push(c);
				continue;
			}
			
			// handle situation ) } ] appears but stack is null. stack.pop() will throw exception.
			if ((c== ')' || c== '}' || c== ']') && stack.size() == 0) {
				return false;
			}
			
			if (c == ')') {
				if ('(' != stack.pop()) {
					return false;
				}
			}
			
			if (c == '}') {
				if ('{' != stack.pop()) {
					return false;
				}
			}
			
			if (c == ']') {
				if ('[' != stack.pop()) {
					return false;
				}
			}
		}

		if (stack.size() != 0) {
			return false;
		}

		return true;
	}
}