LeetCode 94. Binary Tree Inorder Traversal java solution

题目要求:
Given a binary tree, return the inorder traversal of its nodes’ values.

For example:
Given binary tree [1,null,2,3],
1
\
2
/
3
return [1,3,2].

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

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