Leetcode-32- longest valid bracket

subject

Ideas

1. I think we should start with the stack , Anyway, matching parentheses are stacks .
2. Problem solving is mainly to put the index on the stack , Calculate the length of the longest valid bracket according to the index , A lot of sense .

Code

        Stack<Integer> st1 = new Stack<>();
        // Length of the longest substring 
        int max = 0;
        //( Starting position 
        int start=0;
        for(int i=0;i<s.length();i++){
    
            if(s.charAt(i)=='('){
    
                st1.push(i);
            }
            else{
    
                // Meet immediately ) 了   But the stack is empty 
                if(st1.empty()){
    
                    start+=1;
                }
                else{
     // At this time, there is (  It can pop up   If it is empty after pop-up   All matching strings are valid , The length is i-start+1
                    st1.pop();
                    if (st1.isEmpty()) {
    
                        max = Math.max(max,i-start+1);
                    }else {
     // If it's not empty   Explain to st1 Up to the top index of   All parentheses are valid strings   The length is i-st1.peek()
                        max = Math.max(max,i-st1.peek());
                    }
                }
            }
        }
        return max;