preorder, inorder, postorder traversal

preorder, inorder, postorder traversal

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?

Recursive solution

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> preorderTraversal(TreeNode root) {        List<Integer> ret = new ArrayList<Integer>();        helper(root, ret);        return ret;    }        private void helper(TreeNode root, List<Integer> ret) {        if (root == null)            return;                ret.add(root.val);        helper(root.left, ret);        helper(root.right, ret);    }}

Iterative solution

public class Solution {    public List<Integer> preorderTraversal(TreeNode root) {        List<Integer> ret = new ArrayList<Integer>();        if (root == null)            return ret;                    Stack<TreeNode> stack = new Stack<TreeNode>();        stack.push(root);        while (!stack.isEmpty()) {            TreeNode node = stack.pop();            ret.add(node.val);                        if (node.right != null) {                stack.push(node.right);            }                        if (node.left != null) {                stack.push(node.left);            }        }                        return ret;                    }        }