Leetcode: Path Sum I&&II

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.
For 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.

Solve the two problems with bug free.

1. For path sum II, use the feature of C++, pass both by value and reference.
2. Vector::push_back()actually creates a copy of the argument and stores it in the vector