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]

递归

/**

* 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;

        inorder(root, res);

        return res;

    }

    void inorder(TreeNode *root, vector<int> &res){

        if(!root) return;

        if(root->left) inorder(root->left, res);

        res.push_back(root->val);

        if(root->right) inorder(root->right, res);

    }

};