Package g1801_1900.s1866_number_of_ways_to_rearrange_sticks_with_k_sticks_visible

Class Solution

  • public class Solution
extends Object
    1866 - Number of Ways to Rearrange Sticks With K Sticks Visible.

    Hard

    There are n uniquely-sized sticks whose lengths are integers from 1 to n. You want to arrange the sticks such that exactly k sticks are visible from the left. A stick is visible from the left if there are no longer sticks to the left of it.

    • For example, if the sticks are arranged [1,3,2,5,4], then the sticks with lengths 1, 3, and 5 are visible from the left.

    Given n and k, return the number of such arrangements. Since the answer may be large, return it modulo 109 + 7.

    Example 1:

    Input: n = 3, k = 2

    Output: 3

    Explanation: [1,3,2], [2,3,1], and [2,1,3] are the only arrangements such that exactly 2 sticks are visible. The visible sticks are underlined.

    Example 2:

    Input: n = 5, k = 5

    Output: 1

    Explanation: [1,2,3,4,5] is the only arrangement such that all 5 sticks are visible. The visible sticks are underlined.

    Example 3:

    Input: n = 20, k = 11

    Output: 647427950

    Explanation: There are 647427950 (mod 109 + 7) ways to rearrange the sticks such that exactly 11 sticks are visible.

    Constraints:

    • 1 <= n <= 1000
    • 1 <= k <= n

    • Constructor Detail

      • Solution

        public Solution()

    • Method Detail

      • rearrangeSticks

        public int rearrangeSticks​(int n,
                           int k)