Create 1866-number-of-ways-to-rearrange-sticks-with-k-sticks-visible.kt

Author: a93a

kotlin/1866-number-of-ways-to-rearrange-sticks-with-k-sticks-visible.kt

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+// Dp bottom up
+class Solution {
+    fun rearrangeSticks(n: Int, k: Int): Int {
+        val mod = 1_000_000_000 + 7
+        val dp = Array (n + 1) { LongArray (k + 1) }
+        
+        dp[1][1] = 1
+        for (i in 2..n) {
+            for (j in 1..k) {
+                dp[i][j] = (dp[i - 1][j - 1] + ((i - 1) * dp[i - 1][j])) % mod
+            }
+        }
+
+        return dp[n][k].toInt()
+    }
+}
+
+// Recursion + memoization
+class Solution {
+    fun rearrangeSticks(n: Int, k: Int): Int {
+        val mod = 1_000_000_000 + 7
+        val dp = Array (n + 1) { LongArray (k + 1) { -1L } }
+
+        fun dfs(n: Int, k: Int): Long {
+            if (n == k) return 1
+            if (n == 0 || k == 0) return 0
+            if (dp[n][k] != -1L) return dp[n][k]
+
+            dp[n][k] = dfs(n - 1, k - 1) + ((n - 1) * dfs(n - 1, k))
+
+            return dp[n][k] % mod
+        }
+
+        return dfs(n, k).toInt()
+    }
+}