左根右，中序遍历
附上代码：

class Solution {
public:
    vector<int> result;
    vector<int> inorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        result.clear();
        if(root == NULL)
            return result;
        Traversal(root->left);
        result.push_back(root->val);
        Traversal(root->right);
        return result;
    }
    void Traversal(TreeNode * now)
    {
        if(now == NULL)
            return ;
        Traversal(now->left);
        result.push_back(now->val);
        Traversal(now->right);
    }
};