找出第k小的数字-期望时间为O(n)-随机选择排序-源码

最新推荐文章于 2021-05-26 11:35:16 发布

原创最新推荐文章于 2021-05-26 11:35:16 发布 · 1.4k 阅读

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本文深入探讨了随机选择排序算法的实现细节，并通过实例展示了其在整数数组排序中的应用。包括算法原理、关键步骤解析及代码实现。

改正之前随机选择排序的实现：

#include <iostream>  
#include<ctime>
#include<cstdlib>
using namespace std;  
 
void swap(int &a, int &b) {
	int temp = a;
	a = b;
	b = temp;
}
int rand(int low, int high) {
	int size = high - low + 1;
	return low + rand() % size;
}

int random_partition(int *a, int low, int high) {
	int index = rand(low, high);
	swap(a[index], a[low]);
	int key = a[low];
	int i = low;
	for(int j = low + 1; j <= high; j++) {
		if(a[j] <= key) {
			if(i != j) {
				i = i + 1;
				swap(a[i], a[j]);
			}
		}
	}
	swap(a[low], a[i]);
	return i;
}

void random_sort(int *a, int low, int high) {
	if(low < high) {
		int pivot = random_partition(a, low, high);
		random_sort(a, low, pivot - 1);
		random_sort(a, pivot + 1, high);
	}
}
int random_select(int *a, int low, int high, int k) {
	if(k < 1 || k > (high - low + 1))
		return -1;
	int pos = random_partition(a, low, high);
	int m = pos - low + 1;
	if(m == k) {
		return a[pos];
	} else if(k < m) {
		return random_select(a, low, pos - 1, k);
	} else {
		return random_select(a, pos + 1, high, k - m);
	}
}

void main() {
	int a[] = {33, 22, 43, 23, 12, 54, 76, 47, 38, 66};
	int len = sizeof(a) / sizeof(int);
	int i;
	int result = random_select(a, 0, len - 1, 5);
	//random_sort(a, 0, len - 1);
	for(i = 0; i < len; i++)
		cout << a[i] << '\t';
	cout << "result=" << result << endl;
}