归并排序

• **时间复杂度：O(nlogn)**。

• 步骤：

  1. 确定分界点mid=(l+r)。
  2. 归并排序左右两段。
  3. 有序归并，合二为一。

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 10;
int q[maxn], tmp[maxn], n;

void ms(int q[], int l, int r) {
    if (l >= r) return;
    int mid = l + r >> 1;
    ms(q, l, mid), ms(q, mid + 1, r);
    
    int i = l, j = mid + 1, k = 0;
    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 (int 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]);
    
    ms(q, 0, n - 1);
    
    for (int i = 0; i < n; i++) printf("%d ", q[i]);
    return 0;
}