g1801_1900.s1866_number_of_ways_to_rearrange_sticks_with_k_sticks_visible

Class Solution

java.lang.Object

g1801_1900.s1866_number_of_ways_to_rearrange_sticks_with_k_sticks_visible.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 Summary

Constructors

Constructor and Description
Solution()

Method Summary

Modifier and Type	Method and Description
int	rearrangeSticks(int n, int k)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Solution

public Solution()

Method Detail

rearrangeSticks

public int rearrangeSticks(int n,
                           int k)