從數組中選擇第k大的數

package gxu.jsj.wyg;

public class Select {
	public static void main(String[] args) {
		int[] a = {1,5,8,-4,9,6,3,-1,4,7};
		//SortSelect.sort(a);
		SortSelect.show(a);
		int k = 6;
		SortSelect.select_k(a, 0, a.length, k);
		SortSelect.show(a);
		System.out.printf("第%d大的數爲：%d\n",k,SortSelect.K_Num);
	}
}

class SortSelect {
	public static int K_Num;
	
	//使用冒泡排序算法，對數組進行排序
	public static void sort(int[] a) {
		int i,j;
		int len = a.length;
		for(i=0; i<len-1; i++) {
			for(j=i+1; j<len; j++) {
				if(a[i] < a[j]) {
					swap(a,i,j);
				}
			}
		}
	}
	
	//從a中選擇第k大元素,從排好序的數組中進行選擇
	public static int select_k(int[] a,int k) {
		if(k >= a.length) return -10000;
		sort(a);
		
		return a[k];
	}
	
	//從未排序的數組中進行選擇第k大元素,基於快速排序算法
	public static void select_k(int[] a,int start,int end,int k) {		 
		int m = partation(a, start, end);	 
		int index = k - 1;		
		if(index == m) { 
			K_Num = a[m];
			return;
		}else if(index < m) {
			select_k(a,start,m,k);
		} else {
			select_k(a,m+1,end,k);
		}	
	}
	
	private static int partation(int[] a,int start,int end) {
		
		int s = start;
		int t = end-1;
		int k = a[start];
		
		while(s < t) {
			while(k >= a[t] && s < t) 
				t --;
			if(s<t) swap(a,s,t);
			
			while(k <= a[s] && s < t) 
				s ++;
			if(s<t) swap(a,s,t);			
		}
		return s;
	}
	
	public static void swap(int[] a,int x,int y) {
		int temp = a[x];
		a[x] = a[y];
		a[y] = temp;
	}
	
	public static void show(int[] a) {
		for(int i=0; i<a.length; i++) {
			System.out.printf("%d ",i+1);
		}
		System.out.println();
		for(int i=0; i<a.length; i++) {
			System.out.printf("%d ",a[i]);
		}
		System.out.println();
	}
}