Tree BFS - LC314 Binary Tree Vertical Order Traversal
题目:Givenabinarytree,returntheverticalordertraversalofitsnodes'values.(ie,fromtoptobottom,columnbycolumn).Iftwonodesareinthesamerowandcolumn,theordershouldbefromlefttoright.这道题的目的是把树按列输出,以root为其实点按层遍历,