leetcode - 94. Binary Tree Inorder Traversal

class Solution {
public:
    void visit(vector<int> & ret, TreeNode* root)
    {
        if(root->left) visit(ret,root->left);
        ret.push_back(root->val);
        if(root->right) visit(ret,root->right);
        return ;
    }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ret;
        if(root) visit(ret,root);
        return ret;
    }
};