Path Sum II @leetcode

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Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

» Solve this problem

这题要动态存储找到的path node，思路和permutation一样。还是要注意sol.pop_back()在完成了两个递归之后。