Valid Parentheses @LeetCode

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.

	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;
	}
	}

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