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
    \
     2
    /
   3

 

return [1,2,3].

 

/**
 * 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> res = new ArrayList<>();
        if (root != null) {
        	Stack<TreeNode> stack = new Stack<TreeNode>();
        	stack.add(root);
        	while (!stack.isEmpty()) {
        		root = stack.pop();
        		res.add(root.val);
        		if (root.right != null) {
        			stack.push(root.right);
        		}
        		if (root.left != null) {
        			stack.push(root.left);
        		}
        	}
        }
        return res;
    }
}