leetcode 94. 二叉树的中序遍历

给定一个二叉树，返回它的中序遍历。
示例:
输入: [1,null,2,3]
   1
    
     2
    /
   3
输出: [1,3,2]

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        helper(root, res);
        return res;
    }
    void helper(TreeNode* root, vector<int>& res) {
        if (root == nullptr)
            return;
        if (root->left)
            helper(root->left, res);
        res.push_back(root->val);
        if (root->right)
            helper(root->right, res);
    }
};