Divide 1d vector into 2d vector

[Ramees219]

Hi i need divide below vector by 3 into a 2d vector

   vector<int> a = {5,6,8,7,5,4,7,5};

Example

   vector<vector<int> > b = {{5,6,8},{7,5,4},{7,5}};

How can i do that i need help