N个数中第k大的元素

// 3 implement the function : int part(int *a,int low,int high), and implement the function to find the k-st num in a array
	a[low] as the pivot element
	int part(int *a,int low,int high){
		int temp = a[low];
		int i = low ;
		for(int j=low+1;j<=high;j++){
			if(a[j] <= temp){
				i++;
				exchange(a[i],a[j]);
			}
		}
		exchange(a[i],a[low]);
		return i;
	}
	int find_k_st(int *a,int start,int end){
		if(start < end){
			int mid = part(a,start,end);
			if(mid == k-1)
				return a[mid];
			else{
				if(k-1 < mid)
					find_k_st(a,start,mid-1);
				else
					find_k_st(a,mid+1,end);
			}
		}
	}