969. Pancake Sorting

class Solution {
    List<Integer> result = new LinkedList<>();
    public List<Integer> pancakeSort(int[] arr) {
        recursion(arr, arr.length-1);
        return result;
        
    }
    private void recursion(int[] arr, int end){
        if(end==0){
            return;
        }
        int index = max(arr, end);
        if(index!=end){
            result.add(index+1);
            result.add(end+1);
            flip(arr, index);
            flip(arr, end);
        }
        recursion(arr, end-1);
    }
    // return the index of the largest num from [0, end];
    private int max(int[] arr, int end){
        int maxIndex = 0;
        for(int i=1; i<=end; i++){
            maxIndex = arr[i]>arr[maxIndex] ? i:maxIndex;
        }
        return maxIndex;
    }
    
    private void flip(int[] arr, int end){
        int start=0;
        while(start<end){
            int temp = arr[end];
            arr[end]=arr[start];
            arr[start]=temp;
            start++;
            end--;
        }
    }
}