Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes 'values. for example: Given binary tree {1, #, 2/3}, 1 \ return [, 3]. note: Recursive solution is trivial, cocould you do it iteratively? Java code: recursive solution:

/**  * 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) {          // IMPORTANT: Please reset any member data you declared, as          // the same Solution instance will be reused for each test case.          ArrayList<Integer> res = new ArrayList<Integer>();           if(root == null)               return res;           pre(res,root);           return res;      }      public void pre(ArrayList<Integer> res, TreeNode root)       {           res.add(root.val);           if(root.left != null)               pre(res, root.left);           if(root.right != null)               pre(res, root.right);       }  }  

 

Iteratively solution:

/**  * 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) {          // IMPORTANT: Please reset any member data you declared, as          // the same Solution instance will be reused for each test case.          ArrayList<Integer> res = new ArrayList<Integer>();           if(root == null)               return res;           Stack<TreeNode> st = new Stack<TreeNode>();           st.push(root);           while(!st.isEmpty())           {               TreeNode cur = st.peek();               res.add(cur.val);               st.pop();               if(cur.right != null)                   st.push(cur.right);               if(cur.left != null)                   st.push(cur.left);           }           st = null;           return res;      }        }