小米二轮面试编程题：二叉树的前序遍历，要求非递归方式

import java.util.*;
public class Solution {
    ArrayList<Integer> list = new ArrayList<Integer>();
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        if(root == null)
            return list;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode temp = stack.pop();
            list.add(temp.val);
            if(temp.right != null)
                stack.push(temp.right);
            if(temp.left != null)
                stack.push(temp.left);
        }
        return list;
    }
}