和快速排序有点类似，利用快速排序的划分算法，

划分算法见http://blog.csdn.net/buyingfei8888/article/details/8997803

根据int partition(int number[],int start,int end)；返回值为数组下标，大小为index，index左边值均小于number【index】，右边均大于number【index】，若为k-1.则左边值均为所求。

代码：

#include <iostream>

using namespace std;

int partition(int number[],int start,int end){
	int temp=number[start];
	while(start<end){
		while(start<end && number[end]>temp)
			--end;
		if(start < end)
		    number[start++] = number[end];
		while(start<end && number[start]<temp)
			start++;
		if(start<end)
		     number[end--] = number[start];

		number[start]=temp;
	}

	return end;
}

void getLeastNumber(int * input,int start,int end,int * output,int k){
	if(NULL == input || NULL == output || k <=0 || start <0 || end < 1)
		return ;
	int index=partition(input,start,end);
	while(index != k-1){
		if(index > k-1){
			end = index -1;
			index =  partition(input,start,end);
		}
		if(index < k-1){
			start = index + 1;
			index = partition(input , start , end );
		}
	}
	for(int i=0;i<k;i++){
		output[i] = input[i];
		cout<< input[i]<<" ";
	}
	
}


int main(){
	int input[8]={2,1,4,3,99,100,56,909};
	int *output;
	getLeastNumber(input,0,7,output,7);
	return 0;

}