O(n)复杂度实现寻找数组第k小的数（快速选择算法）

非堆排序实现，利用快速排序思想实现的快速选择

package algorithm;

public class Test {

    public int quickSelect(int nums[], int left, int right, int k){
        if (left == right) return nums[left];
        int i = left - 1, j = right + 1, x = nums[left];
        while (i < j){
            do i++; while (nums[i] < x);
            do j--; while (nums[j] > x);
            if (i < j){
                int tmp = nums[i];
                nums[i] = nums[j];
                nums[j] = tmp;
            }
        }
        int lenL = j - left + 1;
        if (lenL >= k) return quickSelect(nums, left, j, k);
        else return quickSelect(nums, j + 1, right, k - lenL);
    }

    public static void main(String[] args) {
        int[] nums = new int[]{0, 1, 0, 3, 12};
        Test test = new Test();
        System.out.println(test.quickSelect(nums, 0, nums.length -  1, 3));
    }
}

图示：