Editorial for DMOPC '14 Contest 2 P4 - Deforestation
Submitting an official solution before solving the problem yourself is a bannable offence.
We will use a prefix sum array here. Let ~P[i]~ be the sum of all masses from element 1 to element ~i~. The sum of all masses from ~i~ to ~j~ can be expressed as ~P[j] - P[i] + mass[i]~. The preprocessing time is linear and each query takes constant time. Python users having a hard time should try the PyPy interpreter. Fast input and output methods should be used if possible.