In a binary tree, a lonely node is a node that is the only child of its parent node. The root of the tree is not lonely because it does not have a parent node.

Given the `root`

of a binary tree, return an array containing the values of all lonely
nodes in the tree. Return the list in any order.

Example 1:

Input: root = [1,2,3,null,4]

Output: [4]

Output: [4]

Explanation: Light blue node is the only lonely node.

Node 1 is the root and is not lonely.

Nodes 2 and 3 have the same parent and are not lonely.

Node 1 is the root and is not lonely.

Nodes 2 and 3 have the same parent and are not lonely.

Example 2:

Input: root = [7,1,4,6,null,5,3,null,null,null,null,null,2]

Output: [6,2]

Output: [6,2]

Explanation: Light blue nodes are lonely nodes. Please remember that order doesn’t
matter, [2,6] is also an acceptable answer.

Example 3:

Input: root = [11,99,88,77,null,null,66,55,null,null,44,33,null,null,22]

Output: [77,55,33,66,44,22]

Output: [77,55,33,66,44,22]

Explanation: Nodes 99 and 88 share the same parent. Node 11 is the root. All other
nodes are lonely.

**Solution**

Create a list to store the values of all lonely nodes. If

`root`

is `null`

, return the list directly.
Do breadth first search. For each node, check its two children. For each
non-empty child, offer the child to the queue for the next step’s search. If
exactly one child is non-empty, then add the non-empty child’s value into
the list.Finally, return the list

class Solution { public List<Integer> getLonelyNodes(TreeNode root) { List<Integer> lonelyNodes = new ArrayList<Integer>();if (root == null) return lonelyNodes;Queue<TreeNode> queue = new LinkedList<TreeNode>(); queue.offer(root);while (!queue.isEmpty()) { TreeNode node = queue.poll(); TreeNode left = node.left, right = node.right;if (left != null && right != null) { queue.offer(left); queue.offer(right); } else if (left != null) { lonelyNodes.add(left.val); queue.offer(left); } else if (right != null) { lonelyNodes.add(right.val); queue.offer(right); } } return lonelyNodes; } }