LeetCode - Binary-Tree-Preorder-Traversal

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题目描述
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].
Note: Recursive solution is trivial, could you do it iteratively?

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

//非递归方式
import java.util.ArrayList;
import java.util.Stack;
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        if(root == null) return result;        
        stack.push(root);
        while(!stack.isEmpty()){
             TreeNode node = stack.pop();
              result.add(node.val);
              if(node.right != null)  stack.push(node.right);
              if(node.left != null) stack.push(node.left);
        }
         return result;
    }
}