从数组中找出第K大的数

 

利用改进的快排方法

public class QuickFindMaxKValue {

    public static void main(String[] args) {
        int[] a = {8, 3, 4, 1, 9, 7, 6, 10, 2, 5};
        System.out.println(findMaxValue(a, 0, a.length - 1, 2));
    }

    private static int findMaxValue(int[] a, int lo, int hi, int maxValueIndex) {
        if (lo >= hi) {
            return -1;
        }
        if (maxValueIndex == a.length) {
            return a[maxValueIndex];
        }
        int pivot = partition(a, lo, hi);
        if (maxValueIndex < pivot) {
            return findMaxValue(a, lo, pivot - 1, maxValueIndex);
        } else if (maxValueIndex > pivot) {
            return findMaxValue(a, pivot + 1, hi, maxValueIndex);
        }
        return pivot;
    }

    private static int partition(int[] a, int lo, int hi) {
        int mid = (lo + hi) / 2;
        if (a[mid] > a[hi]) {
            swapValue(a, mid, hi);
        }

        if (a[lo] > a[hi]) {
            swapValue(a, lo, hi);
        }

        if (a[lo] > a[mid]) {
            swapValue(a, lo, mid);
        }

        int key = a[lo];
        while (lo < hi) {
            while (lo < hi && a[hi] > key) {
                hi--;
            }
            a[lo] = a[hi];
            while (lo < hi && a[lo] < key) {
                lo++;
            }
            a[hi] = a[lo];
        }
        a[lo] = key;
        return lo;
    }

    private static void swapValue(int[] a, int left, int right) {
        if (left == right) {
            return;
        }
        int temp = a[left];
        a[left] = a[right];
        a[right] = temp;
    }
}