LeetCode : Binary Tree Preorder Traversal 二叉树前序遍历 递归 迭代

试题：
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]
Follow up: Recursive solution is trivial, could you do it iteratively?
代码：
递归：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    List<Integer> out = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if(root==null) return out;
        out.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return out;
    }
}

迭代：

/**
 * 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> out = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode tmp = stack.pop();
            if(tmp==null) continue;
            out.add(tmp.val);
            stack.push(tmp.right);
            stack.push(tmp.left);
        }
        return out;
    }
}