002基础算法：归并排序

核心思想：

1. 确定分界点
2. 递归
3. 归并 合二为一

核心模板：

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

完整代码：

#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, 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;
}