Binary Tree Preorder Traversal

Problem

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?

Solution

Recursive Solution

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(res, root);
        return res;
    }

    private void helper(List<Integer> res, TreeNode node) {
        if (node == null) return;
        res.add(node.val);
        helper(res, node.left);
        helper(res, node.right);
    }
}

Iterative Solution using Stack

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

Analysis

Very straightforward solutions above
Preorder: Root - Left - Right