Flip Binary Tree To Match Preorder Traversal — C++

// Time:  O(n)
// Space: O(h)

/**
 * 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> flipMatchVoyage(TreeNode* root, vector<int>& voyage) {
        vector<int> result;
        int i = 0;
        return dfs(root, voyage, &i, &result) ? result : vector<int>{-1};
    }

private:
     bool dfs(const TreeNode* root, vector<int>& voyage,
              int *i, vector<int> *result) {
        if (!root) {
            return true;
        }
        if (root->val != voyage[*i]) {
            return false;
        }
        ++(*i);
        if (root->left && root->left->val != voyage[*i]) {
            result->emplace_back(root->val);
            return dfs(root->right, voyage, i, result) &&
                   dfs(root->left, voyage, i, result);
        }
        return dfs(root->left, voyage, i, result) &&
               dfs(root->right, voyage, i, result);
     }
};