LeetCode in Kotlin

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

Solution

class Solution {
    fun rearrangeSticks(n: Int, k: Int): Int {
        if (k > n || k < 1) {
            return 0
        }
        if (k == n) {
            return 1
        }
        var dp = LongArray(k + 1)
        dp.fill(1)
        var i = 1
        while (i + k <= n) {
            val dp2 = LongArray(k + 1)
            for (j in 1..k) {
                dp2[j] = (dp2[j - 1] + (i + j - 1) * dp[j]) % MOD
            }
            dp = dp2
            i++
        }
        return dp[k].toInt()
    }

    companion object {
        private const val MOD = 1000000007
    }
}

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