merge sort

// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        int[] arr = {2, 1, 3, 1, 2};
        count = 0;
        int[] res = mergeSort(arr, 0, arr.length-1);
        System.out.println(count);
        System.out.println(Arrays.toString(res));
    }
    static int count;
    static int[] mergeSort(int[] arr, int lo, int hi){
        if(lo == hi){
            // int[] bs = new int[1];
            // bs[0] = arr[lo];
            // return bs;
            return new int[]{arr[lo]};
        }
        // if(lo < hi){
            int mid = (lo + hi)/2;
            int[] left = mergeSort(arr, lo, mid);
            int[] right = mergeSort(arr, mid+1, hi);

            int[] res = mergeTwoSortedArray(left, right);
            return res;
        // }else {
            // return new int[]{arr[lo]};
        // }
        
    }
    // cnt = 1
    // {2, 1, 3, 1, 2};
    // 1 2 3       1 2
    // 
    // 
    static int[] mergeTwoSortedArray(int[] left, int[] right){
        int n = left.length;
        int m = right.length;
        
        // 1 2 3 5
        // 2 2 4 
        
        // 1 2 2 2 4 5
        
        
        int[] res = new int[n+m];
        int i = 0;
        int j = 0;
        int k = 0;
        while(i<n && j<m){
            if(left[i]<=right[j]){
                res[k++] = left[i++];
            }else {
                count += n - i;
                res[k++] = right[j++];
            }
        }
        
        while(i<n){
            res[k++] = left[i++];
        }
        while(j<m){
            res[k++] = right[j++];
        }
        return res;
    }
}