class Solution {
public:
    bool isValidSerialization(string preorder) {
        int len = preorder.size(), hole = 1, i = 0;
        while(i < len)
        {
            if(preorder[i] != ',')
                --hole;
            if(hole < 0)return false;
            if(isdigit(preorder[i]))
            {
                while(isdigit(preorder[i]))
                    ++i;
                hole += 2;
            }
            ++i;
        }
        return !hole;
    }
};