## Tuesday, May 17, 2022

### Easy_Question13: Find All The Lonely Nodes

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]
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.

Example 2:

Input: root = [7,1,4,6,null,5,3,null,null,null,null,null,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]
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.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) {
queue.offer(left);
} else if (right != null) {
queue.offer(right);
}
}
return lonelyNodes;
}
}``````

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