快速排序

来源：互联网发布：计算机编程难不难编辑：程序博客网时间：2024/05/29 12:13

数据结构课程上讲过的，今天终于有机会实现啦。

#include <stdio.h>#include <string.h>#include <algorithm>using namespace std;const int N = 1e6 + 5;int a[N];void quick_sort(int l,int r){if(l >= r)return ;int magic = rand() % (r - l) + l + 1;swap(a[l],a[magic]);int i = l, j = r;while(true){while(i < j && a[j] >= a[i])j--;if(i == j)break;swap(a[i++],a[j]);while(i < j && a[i] <= a[j])i++;if(i == j)break;swap(a[i],a[j--]);}quick_sort(l,i-1);quick_sort(j+1,r);}int main(){int n;while(~scanf("%d",&n)){for(int i=0;i<n;i++)scanf("%d",&a[i]);quick_sort(0,n-1);for(int i=0;i<n;i++)printf("%d ",a[i]);puts("");}return 0;}

扩展：找第K大元素。复杂度O(n)，不需要对整个数组进行排序。

发现 STL 有实现，叫 nth_element()，复杂度 O(n)。

default (1)

template <class RandomAccessIterator>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last);

custom (2)

template <class RandomAccessIterator, class Compare>  void nth_element (RandomAccessIterator first, RandomAccessIterator nth,                    RandomAccessIterator last, Compare comp);

再扩展一个叫 partial_sort()，保留前K大，复杂度 O(n * log k)。

efault (1)

template <class RandomAccessIterator>  void partial_sort (RandomAccessIterator first, RandomAccessIterator middle,                     RandomAccessIterator last);

custom (2)

template <class RandomAccessIterator, class Compare>  void partial_sort (RandomAccessIterator first, RandomAccessIterator middle,                     RandomAccessIterator last, Compare comp);

#include <stdio.h>#include <string.h>#include <algorithm>using namespace std;const int N = 1e4 + 5;int a[N];int quick_select(int l,int r,int k){int magic = rand() % (r - l) + l + 1;swap(a[l],a[magic]);int i = l, j = r;while(true){while(i < j && a[j] >= a[i])j--;if(i == j)break;swap(a[i++],a[j]);while(i < j && a[i] <= a[j])i++;if(i == j)break;swap(a[i],a[j--]);}if(k == i - l + 1)return a[i];else if(k < i - l + 1)return quick_select(l,i-1,k);elsereturn quick_select(j+1,r,k-(i-l+1));}int main(){int n,k;while(~scanf("%d%d",&n,&k)){for(int i=0;i<n;i++)scanf("%d",&a[i]);printf("%d\n",quick_select(0,n-1,n+1-k));}return 0;}

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