144. Binary Tree Preorder Traversal.cs

// recursion
public class Solution {
    public IList<int> PreorderTraversal(TreeNode root) {
        var res = new List<int>();
        if (root == null) return res;
        res.Add(root.val);
        if (root.left != null){
            res.AddRange(PreorderTraversal(root.left));
        }
        if (root.right != null) {
            res.AddRange(PreorderTraversal(root.right));
        }
        return res;
    }
}

// iteration
public class Solution {
    public IList<int> PreorderTraversal(TreeNode root) {
        var res = new List<int>();
        if (root == null) return res;
        var stack = new Stack<TreeNode>();
        stack.Push(root);
        while (stack.Count > 0) {
            var node = stack.Pop();
            res.Add(node.val);
            if (node.right != null) stack.Push(node.right);
            if (node.left != null) stack.Push(node.left);
        }
        return res;
    }
}