无序数组求第K大

对于第k大,我们可以用堆排序求前k大,还可以用快速排序的思想,求出第k大,我们会发现,当求出的下角标等于我们想要的k时,就是第k大元素了.

import java.util.Scanner;


public class demo {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		int[] a = {1,4,7,8,9};
		int ans = solve(a,2);
		System.out.println("ans: "+ans);
	}

	private static int solve(int[] a, int k) {
		int l = 0;
		int r = a.length - 1;
		int index = 0;
		while(index!=k){
			index = qucik_sort(a,l,r);
			if(index<k){
				l = index + 1; 
			}else if(index>k){
				r = index - 1;
			}
		}
		return a[index];
	}

	private static int qucik_sort(int[] a, int l, int r) {
		if(l<r){
			int i = l;
			int j = r;
			int temp = a[i];
			while(i<j){
				while(i<j&&a[j]>=temp){
					j--;
				}
				a[i] = a[j];
				while(i<j&&a[i]<=temp){
					i++;
				}
				a[j] = a[i];
			}
			a[i] = temp;
			return i;
		}
		return -1;
	}

}