LeetCode(Trees&Graphs)Binary Tree Inorder Traversal

Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

Follow up: Recursive solution is trivial, could you do it iteratively?

思路：二叉树的中序遍历，最简单的办法就是经典的递归算法。

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

但是题目中不让用递归，希望用迭代解决