/**
     Quick Selection  Time = O(n)
        Quick Selection T(n) = T(n/2) + O(n) = T(n/4) + O(n/2) + O(n) =  O(n + n/2 + n/4 + ...+ 1) = O(2n)
        Quick Sort T(n) = 2T(n/2) + O(n) = 4T(n/4) + 2*O(n/2) + O(n) = logn * O(n) = nlogn
    */
    public static void main(String[] args) {
        Solution sol = new Solution();
        int[] nums = new int[]{-4,6,1,76,2,3,3,5};
        System.out.print(sol.quickSelection(nums, 8));
    }

    public int quickSelection(int[] nums, int k) {
        return partition(nums, 0, nums.length - 1, k);
    }
    private int partition(int[] nums, int left, int right, int k) {
        if (left == right) return nums[left];
        int pivotIndex = getPivot(left, right);
        int pos = quickSelect(nums, pivotIndex, left, right);
        if (pos == k - 1) {
            return nums[pos];
        } else if (pos < k - 1) {
            return partition(nums, pos + 1, right, k);
        } else {
            return partition(nums, left, pos - 1, k);
        }
    }
    private int quickSelect(int[] nums, int pivotIndex, int left, int right) {
        int pivot = nums[pivotIndex];
        swap(nums, pivotIndex, right);
        int i = left; int j = right;
        while (i <= j) {
            if (nums[i] > pivot) {
                i++;
            } else if (nums[j] <= pivot) {
                j--;
            } else {
                swap(nums, i++, j--);
            }
        }
        swap(nums, i, right);
        return i;
    }
    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
    private int getPivot(int left, int right) {
        Random rd = new Random();
        return left + rd.nextInt(right - left + 1);
    }