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?
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public ArrayList<Integer> preorderTraversal(TreeNode root) { ArrayList<Integer> result = new ArrayList<Integer>(); if(root != null){ Stack<TreeNode> sta = new Stack<TreeNode>(); sta.push(root); while(!sta.empty()){ TreeNode aNode = sta.pop(); if(aNode.right != null) sta.push(aNode.right); if(aNode.left != null) sta.push(aNode.left); result.add(aNode.val); } } return result; } }