快速选择

来源：互联网发布：linux系统模拟下载编辑：程序博客网时间：2024/05/17 22:00

《数据结构与算法分析——c语言描述》第七章

算法真的很重要，选择问题是这本书一个循序渐进的例子。这次把复杂度强行降到O（N）。

#include<stdlib.h>  #include<stdio.h>  int RandInt(int i, int j) {int temp;temp = (int)(i + (1.0*rand() / RAND_MAX)*(j - i));return temp;}void getRandomInt(int *A, int n) {for (int i = 0; i < n; i++) {A[i] = i + 1;}for (int i = 1; i < n; i++) {//std::swap(A[i], A[RandInt(0, i)]);    int randAdrr = RandInt(0, i);int t = A[i];A[i] = A[randAdrr];A[randAdrr] = t;}}int a[99999999];typedef int ElementType;void insertionSort(int *a, int n) {int j, p;int temp;for (p = 1; p < n; p++) {temp = a[p];for (j = p; j > 0 && temp < a[j - 1]; j--)a[j] = a[j - 1];a[j] = temp;}}void swap_my(ElementType *a, ElementType *b) {ElementType temp;temp = *a;*a = *b;*b = temp;}ElementType median3(ElementType a[], int left, int right) {int center = (left + right) / 2;if (a[left] > a[center])swap_my(&a[left], &a[center]);if (a[left] > a[right])swap_my(&a[left], &a[right]);if (a[center] > a[right])swap_my(&a[center], &a[right]);swap_my(&a[center], &a[right - 1]);return a[right - 1];}#define CUTOFF (3)void qSelect(ElementType a[], int k, int left, int right) {if (left + CUTOFF <= right) {int i, j;ElementType pivot;pivot = median3(a, left, right);i = left;j = right - 1;while (1) {while (a[++i] < pivot) {}while (a[--j] > pivot) {}if (i < j)swap_my(&a[i], &a[j]);elsebreak;}swap_my(&a[i], &a[right - 1]);//把枢纽元换回来if (k < i+1)//k从1开始,i从0开始，小于枢纽元qSelect(a, k, left, i - 1);else if (k > i+1)qSelect(a, k, i + 1, right);}else {insertionSort(a + left, right - left + 1);//第k大的位于left和right之间，拍一下序}}#define N 99999999int main() {getRandomInt(a, N);int k;scanf("%d", &k);qSelect(a,k,0, N-1);printf("%d", a[k-1]);}

0 0