九大经典算法之归并排序

//循环实现
void merge(int arr[], int l, int m, int r) 
{ 
    int i, j, k; 
    int n1 = m - l + 1; 
    int n2 =  r - m; 

    int L[n1], R[n2]; 

    for (i = 0; i < n1; i++) 
        L[i] = arr[l + i]; 
    for (j = 0; j < n2; j++) 
        R[j] = arr[m + 1+ j]; 

    i = 0; 
    j = 0; 
    k = l; 
    while (i < n1 && j < n2) { 
        if (L[i] <= R[j]) { 
            arr[k] = L[i]; 
            i++; 
        } 
        else{ 
            arr[k] = R[j]; 
            j++; 
        } 
        k++; 
    }   
    while (i < n1) { 
        arr[k] = L[i]; 
        i++; 
        k++; 
    }   
    while (j < n2) { 
        arr[k] = R[j]; 
        j++; 
        k++; 
    } 
} 
  
void merge_sort(int arr[], int l, int r) 
{ 
    if (l < r) 
    { 
        int m = l+(r-l)/2;   
        mergeSort(arr, l, m); 
        mergeSort(arr, m+1, r);  
        merge(arr, l, m, r); 
    } 
} 

//递归实现
void merge(int arr[], int tmp[], int start, int end, int middle) {
    int l, r, s;
    s = start;
    l = start;
    r = middle + 1;
    while (l <= middle && r <= end) {
        if (arr[l] <= arr[r]) tmp[s++] = arr[l++];
        else tmp[s++] = arr[r++];
    }
    while (l <= middle) tmp[s++] = arr[l++];
    while (r <= end) tmp[s++] = arr[r++];
    for (;start <= end;start++)
        arr[start] = tmp[start];
}

void msort(int arr[], int *tmp, int start, int end) {
    int middle;
    if (start < end) {
        middle = (start + end) / 2;
        msort(arr, tmp, start, middle);
        msort(arr, tmp, middle + 1, end);
        merge(arr, tmp, start, end, middle);
    }
}

void merge_sort(int arr[], int n) {
    int *tmp = (int*)malloc(n*sizeof(int));
    msort(a, tmp, 0, n - 1);
    free(tmp);
}