(Arithmetic exercises) - random selection algorithm

"Algorithm Practice" P149
Case code:

#include <stdio.h>
#include <stdlib.h>
#include <ctime>
#include <math.h>
#include <algorithm>
using namespace std;
const int maxn = 100010;
int A[maxn],n;//A存放所有整数，n为其个数
//随机选择主元，对区间[left,right]进行划分 

int randPartition(int A[],int left,int right){
	//生成[left,right]内的随机数p 
	//书上这里似乎少了个)，而且头文件要用math.h
	int p = (round(1.0*rand()/RAND_MAX*(right - left) + left));
	swap(A[p],A[left]);//交换A[P]和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;
}
//随机选择算法，从A[left,right]中找到第K大的数，并进行划分
void randSelect(int A[],int left,int right,int K){
	if(left == right) return;
	int p = randPartition(A,left,right);
	int M = p - left + 1;
	if(K == M) return;
	if(K <M){
		randSelect(A,left,p-1,K);
	}
	else{
		randSelect(A,p + 1,right,K-M);
	}
}

int main(){
	srand((unsigned)time(NULL));
	int sum = 0,sum1 = 0;
	scanf("%d",&n);
	for(int i = 0;i <n;i++){
		scanf("%d",&A[i]);
		sum += A[i];
	}
	randSelect(A,0,n - 1,n/2);
	for(int i = 0;i <n/2;i++){
		sum1 += A[i];
	}
	printf("%d\n",(sum - sum1) - sum1);
	return 0;
}