kt: add «1261. Find Elements in a Contaminated Binary Tree»

URL:	https://leetcode.com/problems/find-elements-in-a-contaminated-binary-tree/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/find-elements-in-a-contaminated-binary-tree.kt

@@ -0,0 +1,28 @@
+class FindElements(root: TreeNode?) {
+    private val seen: MutableSet<Int> = mutableSetOf()
+
+    private fun bfs(root: TreeNode?) {
+        val q = ArrayDeque<TreeNode?>()
+        root?.`val` = 0
+        q.add(root)
+
+        while (q.isNotEmpty()) {
+            val node = q.removeFirst()
+            seen.add(node!!.`val`)
+
+            listOf(node?.left, node?.right)
+                .withIndex()
+                .filter { it.value != null }
+                .forEach { it ->
+                    it.value?.`val` = node!!.`val` * 2 + 1 + it.index
+                    q.add(it.value)
+                }
+        }
+    }
+
+    init {
+        bfs(root)
+    }
+
+    fun find(target: Int): Boolean = seen.contains(target)
+}