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].