Verify Preorder Serialization of a Binary Tree

bool isValidSerialization(string preorder) {    int len = preorder.size();    vector<char> temp;    bool flag = true;    for (int i = 0; i < len; i++) {        if (flag == true) {            temp.push_back(preorder[i]);            flag = false;        }           if (preorder[i] == ',') {            flag = true;            continue;        }        int sz = temp.size();        while (sz > 1 && temp[sz - 1] == '#'&&temp[sz - 2] == '#') {            temp.pop_back();            temp.pop_back();            if (temp.empty()) return false;            temp.pop_back();                    temp.push_back('#');            sz = temp.size();        }    }    return temp.size()==1&&temp[0]=='#';}