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/*
随机选择算法的实现
*/
#include<cstdio>
#include<ctime>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
// 随机划分算法 : 随机选择主元,对A中left right 进行划分
int randPartition(int A[], int left, int right){
int p = round(1.0 * rand() / RAND_MAX * (right - left) + left);
swap(A[p], A[left]);
int temp = A[left];
while(left < right){
while(left < right && A[right] > temp) right --;
A[left] = A[right];
while(left < right && A[left] <= temp) left ++;
A[right] = A[left];
}
A[left] = temp;
return left;
}
int startRandSelect(int A[], int left, int right, int K){
if(left == right) return A[left];
int p = randPartition(A, left, right);
int M = p - left + 1; // 主元一定是第M大
if(M == K){
return A[p];
} else if(K < M){
return startRandSelect(A, left, p - 1, K);
} else {
return startRandSelect(A, p + 1, right, K - M);
}
}
// 从A中返回第K大的数字
int randSelect(int A[], int left, int right, int K){
srand((unsigned)time(NULL));
return startRandSelect(A, left, right, K);
}
int main(){
int m[5] = {3,424 ,54 ,23, 1};
printf("%d", randSelect(m, 0, 4, 7));
return 0;
}