二叉树的前序遍历

题目描述：

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

思路：上一题用的是栈，和列表头插法，这一题用的是栈和列表尾插法

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        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;
    }
}