Given a binary tree, return the preorder traversal of its nodes‘ values.
For example:
Given binary tree {1,#,2,3},
1
2
/
3
return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?
def preorder_traversal(root) ans, stack = [], [root] while not stack.empty? if node = stack.pop ans << node.val stack << node.right << node.left end end ans end
Leetcode 144 Binary Tree Preorder Traversal
原文：http://www.cnblogs.com/lilixu/p/4575086.html