归并的时间复杂度

循环递归，递归深度为lgn。
每层递归，所有递归段长度之和为n。
综合可知，总时间复杂度为nlgn。

促背模板:归并排序

#include<iostream>

using namespace std;

const int N = 1e6 + 10;

int n;
int q[N], tmp[N];
// 递归判断l>=r
// 归并三步：二分段，前递归二分后的段，递归后归并合并段到辅助数组
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(j <= mid ) tmp[k++] = q[i++];
    while(j <= r) tmp[k++] = q[j++];

    for(i = l, j = 0; i <= r; i++, j++) q[i] = tmp[j];
}

int main()
{
    scanf("%d", &n);
    for(int i = 0; i < n; i++) scanf("%d", &q[i]);

    merge_sort(q, 0, n - 1);

    for(int i = 0; i < n; i++) printf("%d ", q[i]);

    return(0);
}