94. Binary Tree Inorder Traversal

QUESTION:

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].

EXPLANATION:

二叉树的中序遍历，其实也没有什么好说的。

SOLUTION:

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        inorderTraversalHelper(root,result);
        return result;
    }
    public void inorderTraversalHelper(TreeNode root,ArrayList<Integer> set){
        if(root==null) return;
        inorderTraversalHelper(root.left,set);
        set.add(root.val);
        inorderTraversalHelper(root.right,set);
    }
}
--> return <--