leetcode--Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

The idea in the following program is not normal

public class Solution {
	/**
	 * This is a fundamental problem on implementation of Stack
	 * 
	 * @param s --the string of parentheses
	 * @return maximum number of valid parentheses
	 * @author Averill Zheng
	 * @version 2014-06-02
	 * @since JDK 1.7
	 */
	public int longestValidParentheses(String s) {
		int validParen = 0;
		Stack<String> paren = new Stack<String>();
		paren.push(Integer.toString(0));
		
		int length = s.length();
		for(int i = 0; i < length; ++i){			
			char currentpare = s.charAt(i);
			if(currentpare == '(')
				paren.push("(");
			else{
				String topString = paren.pop();				
				if(topString.equals(")"))  //topStirng is ")"
					paren.push(")");
				//toString is an integer. In this case, the string in the stack maybe
				// integer, ( integer ....
				else if(!topString.equals("(")){ 
					if(!paren.isEmpty() && paren.peek().equals("(")){
						paren.pop();
						int tempNumber = Integer.parseInt(topString) + 2;
						if(!paren.isEmpty() && !paren.peek().equals("(") && !paren.peek().equals(")"))
							tempNumber += Integer.parseInt(paren.pop());
						validParen = (validParen < tempNumber) ? tempNumber : validParen;
						paren.push(Integer.toString(tempNumber));
					}
					else //no "(" can matches current ")"
						paren.push(")");
				}
				else{  //topString is "("
					int tempNumber = 2;
					if(!paren.isEmpty()){
						String secondTop = paren.peek();
						if(!secondTop.equals("(") && !secondTop.equals(")"))
							tempNumber += Integer.parseInt(paren.pop());
					}
					validParen = (validParen < tempNumber) ? tempNumber : validParen;
					paren.push(Integer.toString(tempNumber));
				}
			}
		}
		return validParen;
    }
}