Path Sum

Given a binary tree and a sum,
determine if the tree has a root-to-leaf path such that adding up all the values
along the path equals the given sum.
Example
Given the below binary tree and sum = 22,
      5
     / \
    4   8
   /     / \
  11  13  4
 /  \           \
7    2         1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

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