LeetCode

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;    }}