(二叉树 递归) leetcode94. 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?

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和leetcode 144. Binary Tree Preorder Traversal 几乎相似。题目要求我们尽量用迭代求解，不过，我不太会，所以用递归。

C++代码：

/**
 * 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> vec;
        inorder(root,vec);
        return vec;
    }
    void inorder(TreeNode *root,vector<int> &vec){
        if(!root) return;
        inorder(root->left,vec);
        vec.push_back(root->val);
        inorder(root->right,vec);
    }
};