归并排序

模板

const int N = 1e6 + 10;
int a[N], tmp[N];  // 定义一个缓存数值

void merge_sort(int q[], int l, int r)
{
    if(l >= r) return;
    int mid = l + r >> 1;
    merge_sort(q, l, mid), merge_sort(q, mid + 1, r);
    int k = 0, i = l, j = mid + 1;
    while(i <= mid && j <= r)
    {
        if(q[i] <= q[j]) tmp[k ++ ] = q[i ++ ];
        else tmp[k ++ ] = q[j ++ ];
    }
    while(i <= mid) tmp[k ++ ] = q[i ++ ];
    while(j <= r) tmp[k ++ ] = q[j ++ ];
    
    for(k = 0, i = l; i <= r; k ++ , i ++ ) q[i] = tmp[k];
}