971. Flip Binary Tree To Match Preorder Traversal

class Solution {
public:
    vector<int> flipMatchVoyage(TreeNode* root, vector<int>& voyage) {
        if (flip(root, voyage)) return res;
        // If failed to flip, clear exist element in res
        res.clear();
        res.push_back(-1);
        return res;
    }
    
    bool flip(TreeNode* root, vector<int>& v) {
        if (root == NULL) return true;
        if (root->val != v[i]) return false;
        
        ++i;
        if (root->left && root->left->val == v[i]) {
            return flip(root->left, v) && flip(root->right, v);
        } else if (root->right && root->right->val == v[i]) {
            // Flip needs two children not null ?? 题目划线部分重点！！
            if (root->left)
                res.push_back(root->val);
            return flip(root->right, v) && flip(root->left, v);
        }
        // It may no children
        return !root->left && !root->right;
    }
    
    vector<int> res;
    int i=0;
};