kt: add «1123. Lowest Common Ancestor of Deepest Leaves»

URL:	https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/lowest-common-ancestor-of-deepest-leaves.kt

@@ -0,0 +1,18 @@
+class Solution {
+    private fun dfs(node: TreeNode?): Pair<TreeNode?, Int> =
+        when {
+            node == null -> null to 0
+            else -> {
+                val (left, leftDepth) = dfs(node.left)
+                val (right, rightDepth) = dfs(node.right)
+
+                when {
+                    leftDepth < rightDepth -> right to rightDepth + 1
+                    leftDepth > rightDepth -> left to leftDepth + 1
+                    else -> node to leftDepth + 1
+                }
+            }
+        }
+
+    fun lcaDeepestLeaves(root: TreeNode?): TreeNode? = dfs(root).let { (node, _) -> node }
+}