Day 145.java

class Solution {
    
    static int inversionCount(int arr[]) {
        if (arr.length < 2) return 0;
        return mergeSort(arr, 0, arr.length - 1);
    }
    
    static int mergeSort(int arr[], int left, int right) {
        int count = 0;
        
        if (left < right) {
            int mid = left + (right - left) / 2;
            
            count += mergeSort(arr, left, mid);
            count += mergeSort(arr, mid + 1, right);
            count += merge(arr, left, mid, right);
        }
        
        return count;
    }
    
    static int merge(int arr[], int left, int mid, int right) {
        int temp[] = new int[right - left + 1];
        int i = left;
        int j = mid + 1;
        int k = 0;
        int count = 0;
        
        while (i <= mid && j <= right) {
            if (arr[i] <= arr[j]) {
                temp[k++] = arr[i++];
            } else {
                temp[k++] = arr[j++];
                count += (mid - i + 1);
            }
        }
        
        while (i <= mid) {
            temp[k++] = arr[i++];
        }
        
        while (j <= right) {
            temp[k++] = arr[j++];
        }
        
        for (i = left, k = 0; i <= right; i++, k++) {
            arr[i] = temp[k];
        }
        
        return count;
    }
}