编辑代码

#include <stdio.h>

int merge(int arr[], int left[], int leftSize, int right[], int rightSize) {
    int i = 0, j = 0, k = 0;
    int count = 0; 
    
    while (i < leftSize && j < rightSize) {
        if (left[i] <= right[j]) {
            arr[k] = left[i];
            i++;
        } else {
            arr[k] = right[j];
            j++;
            count += leftSize - i; // 统计逆序对数量
        }
        k++;
    }
    
    while (i < leftSize) {
        arr[k] = left[i];
        i++;
        k++;
    }

    while (j < rightSize) {
        arr[k] = right[j];
        j++;
        k++;
    }
    
    return count;
}

int mergeSort(int arr[], int size) {
    if (size <= 1) {
        return 0;
    }

    int mid = size / 2;
    int left[mid];
    int right[size - mid];

    for (int i = 0; i < mid; i++) {
        left[i] = arr[i];
    }
    for (int i = mid; i < size; i++) {
        right[i - mid] = arr[i];
    }
    
    int count = 0;
    count += mergeSort(left, mid);
    count += mergeSort(right, size - mid);
    count += merge(arr, left, mid, right, size - mid);
    
    return count;
}

int main() {
    int arr[] = {2, 4, 3, 1, 5, 6};
    int n = sizeof(arr) / sizeof(arr[0]);
    
    int count = mergeSort(arr, n);
    
    printf("逆序对的个数是：%d\n", count);
    
    return 0;
}