P4868 Preprefix sum

The prefix sum is defined as $S_i=\sum_{k=1}^i a_k$. The preprefix sum (preprefix sum) takes $S_i$ as the original sequence and computes the prefix sum again. Let the $i$-th value of this second prefix sum be $SS_i$. Given a sequence $a_1, a_2, \cdots, a_n$ of length $n$, there are two types of operations: 1. `Modify i x`: Change $a_i$ to $x$. 2. `Query i`: Query $SS_i$.

The first line contains two integers $N, M$, representing the length of the sequence and the number of operations. The next line contains $N$ numbers, the given sequence $a_1, a_2, \cdots, a_n$. The next $M$ lines each contain one operation, with the format described above.

For each query operation, output one line containing the value of $SS_i$ being queried.

Constraints: $1\le N,M\le 10^5$, and at any time $0\le A_i\le 10^5$. Translated by ChatGPT 5