144. Binary Tree Preorder Traversal

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList();
        Stack<TreeNode> s = new Stack<>();        
        if(root != null) s.push(root);
        
        while(!s.isEmpty()){
            TreeNode p = s.pop();
            res.add(p.val);
            //注意stack是last in first out，所以先push right
            if(p.right != null) s.push(p.right);            
            if(p.left != null) s.push(p.left);
        }
        return res;
    }    
}