快排
分治
-
确定分界点,下标中点
-
递归左、右
-
合并归并 难点 时间复杂度 on
特点
- 不稳定的(除非变成二元组)
#include <iostream>
using namespace std;
const int N = 1e6 + 10;
int n;
int q[N];
void quick_sort(int q[], int l, int r){
if (l >= r) return;
//如果递归是j,此处不能写r,否则有边界问题。
int x = q[l], i = l - 1, j = r + 1;
while (i < j ){
do i ++ ; while (q[i] < x);
do j -- ; while (q[j] > x);
if(i < j) swap(q[i], q[j]);
}
quick_sort(q, l, j);
quick_sort(q, j + 1, r);
}
int main(){
scanf("%d", &n);
for (int i = 0; i < n; i++) scanf("%d", &q[i]);
quick_sort(q, 0, n - 1);
for (int i = 0; i < n; i++) printf("%d ", q[i]);
return 0;
}
交换位置
if(i < j) {
int temp;
temp = q[i];
q[j] = q[i];
q[i] = temp;
}
// swap(q[i], q[j]);
如果将递归的j换成i,会有边界问题
#include <iostream>
using namespace std;
const int N = 1e6 + 10;
int n;
int q[N];
void quick_sort(int q[], int l, int r){
if (l >= r) return;
//如果递归是i,此处不能写l,q[l]否则有边界问题。可以写q[r]、q[(l+r+1)/2]
int x = q[(l + r + 1)/ 2], i = l - 1, j = r + 1;
while (i < j ){
do i ++ ; while (q[i] < x);
do j -- ; while (q[j] > x);
if(i < j) {
int temp;
temp = q[i];
q[j] = q[i];
q[i] = temp;
}
// swap(q[i], q[j]);
}
quick_sort(q, l, i - 1);
quick_sort(q, i, r);
}
int main(){
scanf("%d", &n);
for (int i = 0; i < n; i++) scanf("%d", &q[i]);
quick_sort(q, 0, n - 1);
for (int i = 0; i < n; i++) printf("%d ", q[i]);
return 0;
}
2
1 2