3.归并排序

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 const int N = 100010;
 4 int a[N];
 5 int tmp[N];
 6 void merge_sort(int l, int r) {
 7     if (l >= r) { //区间里没数或只有一个的话 
 8         return;
 9     }
10     //int mid = (l + r) / 2;
11     int mid = (l + r) >> 1; 
12     merge_sort(l, mid);
13     merge_sort(mid + 1, r);
14     int k = 0, i = l, j = mid + 1;
15     //k表示当前合并已经有了多少个数了 
16     while (i <= mid && j <= r) {
17         if (a[i] <= a[j]) {
18             tmp[k++] = a[i++];
19         } else {
20             tmp[k++] = a[j++];
21         }
22     }
23     while (i <= mid) {
24         tmp[k++] = a[i++];
25     }
26     while (j <= r) {
27         tmp[k++] = a[j++];
28     }
29     for (int i = l, j = 0; i <= r; i++, j++) {
30         a[i] = tmp[j];
31     }
32 }
33 int main () {
34     int n;
35     cin >> n;
36     for (int i = 0; i < n; i++) {
37         cin >> a[i];
38     }
39     merge_sort(0, n - 1);
40     for (int i = 0; i < n; i++) {
41         cout << a[i] << " "; 
42     }
43     return 0;
44 }