查找第K大的元素

题目：牛客网

解题思路：

先排序，在输出第三大的数值。使用了快排

 

package offer;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;


public class Main {
	public static int quickSort(int[] data, int k){
		 int begin = 0, end = data.length - 1;
	        int pos = 0;
	        while (begin <= end) {
	            pos = partion(data, begin, end);
	            if (pos == k - 1) {
	            	return data[pos];
	            } else if (pos > k - 1) {
	                end = pos - 1;
	            } else {
	                begin = pos + 1;
	            }
	        }
	        return -1;
	}
	
	public static int partion(int[] data, int begin, int end){
		int temp = data[begin];
        while (begin < end) {
            while (begin < end && data[end] <= temp) {
                end--;
            }
            swap(data, begin, end);
            while (begin < end && data[begin] > temp) {
                begin++;
            }
            swap(data, begin, end);
        }
        return begin;
	}
	
	public static void swap(int[] arr, int i, int j) {
        if (arr == null || i >= arr.length || j >= arr.length || i < 0 || j < 0) {
            return;
        }
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner in = new Scanner(System.in);
		while(in.hasNext()){
			
			String str = in.next();
			String[] s = str.substring(1,str.length()-1).split(",");
			int[] arr = new int[s.length];
			for(int i =0 ; i < s.length; i++){
				arr[i] = Integer.valueOf(s[i]);
			}
			//直接调用sort方法，基于快排的
		    Arrays.sort(arr);
		    System.out.println(arr[arr.length-3]);
		    //自己写的快排方法
		    int result = quickSort(arr,3);
		    System.out.println(result);
		    
		}

	}

}