Longest Valid Parentheses Java

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.

 * A Valid Parentheses pair contain only one left and right bracket.
* 1. store index of '(' in the stack
* 2. check stack whether is empty when ')' was found:
*  two case as following:
*    2.1 stack is not empty-> a valid pair was matched
*      two more case as follow that check whether stack is empty or not 
*      2.1.1: stack is empty -> calculate maxLength=Math.max(maxLength, i-lastIndex);
*      2.1.2  stack is not empty-> maxLength=Math.max(maxLength,i-stack.peek()); 
*    2.2 stack is empty -> not pair matched, updated lastIndex(last index of impartial ')' bracket)

public class Solution {
    public int longestValidParentheses(String s) {
        if(s.isEmpty()) return 0;
	     Stack<Integer> stack=new Stack<Integer>();
	    int maxLength=0;
	     int lastIndex=-1;
		 for(int i=0;i<s.length();i++){
			 char c=s.charAt(i);
			 //find left bracket, 
			 if(c=='('){
				 stack.push(i);
			 }else{
				 if(c==')'){
					 if(!stack.isEmpty()){
						 stack.pop();
						 if(stack.isEmpty()){	 //The entire valid pair group was found;
							//and calculate length of pair group 
							 maxLength=Math.max(maxLength, i-lastIndex);
						 }else{ 
							 //There is more '(' left in stack
							 //update maxLength with counting number of pair so far
							 maxLength=Math.max(maxLength, i-stack.peek());
						 }
					 }else{
						 // last index of impartial ')' bracket
						 lastIndex=i;
					 }
				 }
			 }
		 }
	 return maxLength;   
    }
}