add one more vary solution to 236

Author: Li Li

top-interview-questions/236. Lowest Common Ancestor of a Binary Tree.cs

@@ -7,4 +7,36 @@ public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
         if (left != null && right != null) return root;
         return left == null ? right : left;
     }
+}
+
+// if there are parent for each node, the solution can be changed
+public ParentTreeNode lowestCommonAncestorII(ParentTreeNode root,
+                                                ParentTreeNode A,
+                                                ParentTreeNode B) {
+    ArrayList<ParentTreeNode> pathA = getPath2Root(A);
+    ArrayList<ParentTreeNode> pathB = getPath2Root(B);
+    
+    int indexA = pathA.size() - 1;
+    int indexB = pathB.size() - 1;
+    
+    ParentTreeNode lowestAncestor = null;
+    while (indexA >= 0 && indexB >= 0) {
+        if (pathA.get(indexA) != pathB.get(indexB)) {
+            break;
+        }
+        lowestAncestor = pathA.get(indexA);
+        indexA--;
+        indexB--;
+    }
+    
+    return lowestAncestor;
+}
+
+private ArrayList<ParentTreeNode> getPath2Root(ParentTreeNode node) {
+    ArrayList<ParentTreeNode> path = new ArrayList<>();
+    while (node != null) {
+        path.add(node);
+        node = node.parent;
+    }
+    return path;
 }