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.

Input: root =[1,2,3,null,4]Output: [4]Explanation: Light blue node is the only lonely node.Node 1is the root and is not lonely.Nodes 2 and 3 have the same parent and are not lonely.
Example 2:
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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:
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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 11is the root.All other nodes are lonely.
Constraints:
The number of nodes in the tree is in the range [1, 1000].
A node is lonely if it is the only child of its parent. We can traverse the tree using DFS, and for each node, check if it is the only child of its parent (i.e., its parent has only one child).