1
/ \
2 3
/ \ / \
4 5 6 7
\ \
8 9
The output of print this tree vertically will be:
4
2
1 5 6
3 8
7
9
看不懂这个例子
http://stackoverflow.com/questions/20521098/print-a-tree-vertically
>To understand what's same vertical line, we need to define horizontal distances first. If two nodes have the same Horizontal Distance (HD), then they are on same vertical line. The idea of HD is simple. HD for root is 0, a right edge (edge connecting to right subtree) is considered as +1 horizontal distance and a left edge is considered as -1 horizontal distance. For example, in the below tree, HD for Node 4 is at -2, HD for Node 2 is -1, HD for 5 and 6 is 0 and HD for node 7 is +2.
关键点是理解HD，垂直距离。
1(0)
/ \
2 (-1) 3(1)
/ \ / \
4(-2)5(0) 6(0) 7(2)
\ \
8(1) 9(3)
然后排序输出
-2 4
-1 2
0 1 5 6
1 3 8
2 7
3 0
http://www.geeksforgeeks.org/print-binary-tree-vertical-order/
你这是典型的中序遍历（LDR），中序遍历首先遍历左子树，然后访问根结点，最后遍历右子树。在遍历左、右子树时，仍然先遍历左子树，再访问根结点，最后遍历右子树。