Binary Tree Preorder Traversal

Given the root of a binary tree, return the preorder traversal of its nodes' values.

image

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public IList<int> PreorderTraversal(TreeNode root) {
        IList<int> items = new List<int>();
        if (root == null)
            return items;

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.Push(root);
        while (stack.Count > 0)
        {
            TreeNode node = stack.Pop();
            items.Add(node.val);
            if (node.right != null)
                stack.Push(node.right);
            if (node.left != null)
                stack.Push(node.left);
        }

        return items;
    }
}