[leetcode] Binary Tree Preorder Traversal

https://leetcode.com/problems/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> ans = new ArrayList<Integer>();
		if (root == null) {
			return ans;
		}
        Stack<TreeNode> rs = new Stack<TreeNode>();
		TreeNode p = root;
		
		while(p != null) {
		    ans.add(p.val);
		    
		    if(p.right != null) {
		        rs.push(p.right);
		    }
		    
		    p = p.left;
		    if(p == null && !rs.isEmpty()) {
		        p = rs.pop();
		    }
		}
		
		return ans;
    }
}