java: add «2096. Step-By-Step Directions From a Binary Tree Node to Another»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/step-by-step-directions-from-a-binary-tree-node-to-another.java

@@ -0,0 +1,70 @@
+class Solution {
+  private TreeNode getLowestCommonAncestor(TreeNode node, int key1, int key2) {
+    if (node == null) {
+      return null;
+    }
+
+    if (node.val == key1 || node.val == key2) {
+      return node;
+    }
+
+    var left = getLowestCommonAncestor(node.left, key1, key2);
+    var right = getLowestCommonAncestor(node.right, key1, key2);
+
+    if (left == null) {
+      return right;
+    }
+
+    if (right == null) {
+      return left;
+    }
+
+    return node;
+  }
+
+  private record Path(boolean found, StringBuilder path) {}
+
+  private Path getPath(TreeNode node, int key, StringBuilder path) {
+    if (node == null) {
+      return new Path(false, path);
+    }
+
+    if (node.val == key) {
+      return new Path(true, path);
+    }
+
+    // go left
+    path.append('L');
+    var leftPath = getPath(node.left, key, path);
+    if (leftPath.found) {
+      return leftPath;
+    }
+    path.deleteCharAt(path.length() - 1);
+
+    // go right
+    path.append('R');
+    var rightPath = getPath(node.right, key, path);
+    if (rightPath.found) {
+      return rightPath;
+    }
+    path.deleteCharAt(path.length() - 1);
+
+    return new Path(false, path);
+  }
+
+  public String getDirections(TreeNode root, int startValue, int destValue) {
+    var commonAncestor = getLowestCommonAncestor(root, startValue, destValue);
+    var pathToStart = getPath(commonAncestor, startValue, new StringBuilder());
+    var pathToDest = getPath(commonAncestor, destValue, new StringBuilder());
+
+    var directions = new StringBuilder();
+
+    // gotta go up from the start
+    directions.repeat('U', pathToStart.path.length());
+
+    // and then follow the directions down
+    directions.append(pathToDest.path);
+
+    return directions.toString();
+  }
+}