归并排序算法

①确定分界点 mid = (l+r)/2

②递归排序left right 左右两个数组

③归并排序 合二为一 

#include <iostream>
using namespace std;
const int N = 1e6+10;
 
int n;
int q[N], tmp[N];
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(i = l, k = 0; i <= r; i++, k++) q[i] = tmp[k];
}
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;
}