1.[L, R] => [L, mid], [mid + 1, R]
2.递归排序[L, mid]和[mid + 1, R]
3.归并, 将左右两个有序序列合并成一个有序序列

#include<bits/stdc++.h>
using namespace std;

const int N = 1e5 + 10;

int n;
int a[N], tmp[N];

void merge_sort(int a[], int l, int r){
    if(l >= r) return;

    int mid = l + r >> 1;

    merge_sort(a, l, mid);
    merge_sort(a, mid + 1, r);

    int i = l, j = mid + 1, k = 0;

    while(i <= mid && j <= r){
        if(a[i] <= a[j]) tmp[k++] = a[i++];
        else
            tmp[k++] = a[j++];
    }

    while(i <= mid) tmp[k++] = a[i++];
    while(j <= r)   tmp[k++] = a[j++];

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

int main(){

    cin >> n;
    for(int i = 0; i < n; ++ i)
        scanf("%d", &a[i]);

    merge_sort(a, 0, n-1);

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

    return 0;
}