Your solution is brute, you are literally doing what the query asks you to do. In particular, lines $$$46$$$ and $$$47$$$ are too slow ( they are $$$O(|a|)$$$ which can be $$$O(N)$$$ ). So, the whole complexity becomes $$$O(Q*N)$$$.

You need to solve the problem, without updating the original string each time ( atleast I don't know a fast way to update the original string after each query ).