kt: add «2097. Valid Arrangement of Pairs»

URL:	https://leetcode.com/problems/valid-arrangement-of-pairs/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/valid-arrangement-of-pairs.kt

@@ -0,0 +1,56 @@
+class Solution {
+    private data class Graph(
+        val neighbors: Map<Int, ArrayDeque<Int>>,
+        val ingoing: Map<Int, Int>,
+        val outgoing: Map<Int, Int>,
+    )
+
+    private fun constructGraph(pairs: Array<IntArray>): Graph {
+        val neighbors = mutableMapOf<Int, ArrayDeque<Int>>()
+        val ingoing = mutableMapOf<Int, Int>()
+        val outgoing = mutableMapOf<Int, Int>()
+
+        pairs.forEach { pair ->
+            val (left, right) = pair[0] to pair[1]
+
+            neighbors.getOrPut(left) { ArrayDeque<Int>() }.add(right)
+            outgoing[left] = 1 + outgoing.getOrDefault(left, 0)
+            ingoing[right] = 1 + ingoing.getOrDefault(right, 0)
+        }
+
+        return Graph(neighbors, ingoing, outgoing)
+    }
+
+    private fun findStart(g: Graph): Int =
+        g.outgoing.keys.firstOrNull {
+            g.outgoing[it] == 1 + g.ingoing.getOrDefault(it, 0)
+        } ?: g.outgoing.keys.first()
+
+    private fun visit(
+        g: Graph,
+        path: MutableList<Int>,
+        u: Int,
+    ) {
+        val neighbors = g.neighbors.getOrDefault(u, null)
+        while (neighbors != null && neighbors.isNotEmpty()) {
+            val v = neighbors.removeFirst()
+            visit(g, path, v)
+        }
+
+        path.add(u)
+    }
+
+    fun validArrangement(pairs: Array<IntArray>): Array<IntArray> {
+        val g = constructGraph(pairs)
+        val start = findStart(g)
+
+        val path = mutableListOf<Int>()
+        visit(g, path, start)
+
+        path.reverse()
+
+        return Array(path.size - 1) { i ->
+            intArrayOf(path[i], path[i + 1])
+        }
+    }
+}