Add Lowest Common Ancestor

Author: marttp

kotlin/Trees/LowestCommonAncestor.kt

@@ -0,0 +1,40 @@
+import ds.TreeNode
+
+/*
+    Definition of TreeNode:
+
+    data class TreeNode(
+        var value: Int,
+        var left: TreeNode? = null,
+        var right: TreeNode? = null
+    )
+*/
+
+fun lowestCommonAncestor(root: TreeNode?, p: TreeNode?, q: TreeNode?): TreeNode? {
+    var lca: MutableList<TreeNode?> = mutableListOf(null)
+    dfs(root, p, q, lca)
+    return lca[0]
+}
+
+fun dfs(node: TreeNode?, p: TreeNode?, q: TreeNode?, lca: MutableList<TreeNode?>): Boolean {
+    // Base case: a null node is neither 'p' nor 'q'.
+    if (node == null) {
+        return false
+    }
+    val nodeIsPOrQ = node == p || node == q
+    // Recursively determine if the left and right subtrees contain 'p'
+    // or 'q'.
+    val leftContainsPOrQ = dfs(node.left, p, q, lca)
+    val rightContainsPOrQ = dfs(node.right, p, q, lca)
+    // If two of the above three variables are true, the current node is
+    // the LCA.
+    if ((nodeIsPOrQ && leftContainsPOrQ) ||
+                    (nodeIsPOrQ && rightContainsPOrQ) ||
+                    (leftContainsPOrQ && rightContainsPOrQ)
+    ) {
+        lca[0] = node
+    }
+
+    // Return true if the current subtree contains 'p' or 'q'.
+    return nodeIsPOrQ || leftContainsPOrQ || rightContainsPOrQ
+}