BFS:

if (root == NULL){
    return;
}

queue<int> Q;
Q.push(root);

vector<int> value;
value.push_back(root->val);


while(!Q.empty()){

    int n = Q.size();

    for (int i = 0; i < n; i++){
        TreeNode* node = Q.front();
        Q.pop();

        if (node!=NULL){
            values.push_back(Q->val);
            Q.push(Q->left);
            Q.push(Q->right);
        }
    }
}
return;