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?
与94题很类似，94题是inorder。
用stack，先将左子树push进stack并存进结果的list里。然后到null的时候指向右子数。
public IList<int> PreorderTraversal(TreeNode root) { var res = new List<int>(); var stack = new Stack<TreeNode>(); while(root != null || stack.Count() > 0) { while(root != null) { res.Add(root.val); stack.Push(root); root = root.left; } TreeNode a = stack.Pop(); root = a.right; } return res; }
144. Binary Tree Preorder Traversal
原文：http://www.cnblogs.com/renyualbert/p/5860907.html