解开的都是套路: Facebook脸家 - Lexicographical path
Lexicographical path
5 / \ 3 2 / \ \ 2 4 4 \ 1 FB 面经，自底向上的 path 有【1,4,2,5】，【4,3,5】，【2,3,5】，要求按自底向上的 lexicographical order 返回排序的 path，比如在这里是 【1,4,2,5】, 【2,3,5】,【4,3,5】
Analysis: Post order, for each node keep left & right paths, merge 2 into one single path.
Complexity: O(nlgn), each node has at most lgn node in the path.
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