快排求topk问题

快排思想求topk问题

	public static void main(String[] args) {
//        int[] nums = new int[]{5, 3, 2, 10, 8, 1, 4, 9, 3, 12};
        int[] nums = new int[]{5, 3, 2, 7, 8, 1, 4, 9, 6, 10};

        int top = 2;
        int topk = topk(nums, 0, nums.length -1, top - 1);

        System.out.print("\t" + topk);
    }

    public static int topk(int[] nums, int start, int end, int k) {
        int target = nums[start];
        int i = start;  //不能+1,否则会直接跳过比较
        int j = end;

        while (i < j) {
            while (i < j && nums[j] >= target) j--;  // 因为target是从左开始，所以此处应该从右开始，确保i>j不会出现
            while (i < j && nums[i] <= target) i++;
            if (i < j) swap(nums, i, j);
        }
        swap(nums, start, i);

        if (i < k) return topk(nums, i + 1, end, k);
        else if (i > k) return topk(nums, start, i - 1, k);
        else return nums[i];
    }

    public static void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }