merge_sort 【归并排序】

归并排序

• 归并排序也是分治的思想，但是它的方式是跟快排不太一样
• 归并是先找到中间位置，然后左右递归两边，最后两两将有序之后的数组进行归并
  

时间复杂度：O(nlogn)

模板题：

#include<iostream>
#include<algorithm>

using namespace std;

const int N = 1e5 + 10;
int q[N];
int tmp[N]; //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 (i <= mid) tmp[k++] = q[i++];
    while (j <= r) tmp[k++] = q[j++];
    
    //将tmp数组中的元素放回到原数组中
    for (int i = l, j = 0; i <= r; i++, j++) q[i] = tmp[j];
}

int main() {
    int n;
    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;
}