C, how to put number split over an array, into int

Lets say I have char array[10] and [0] = '1', [1] = '2', [2] = '3', etc.

How would i go about creating (int) 123 from these indexes, using C?

I wish to implement this on an arduino board which is limited to just under 2kb of SRAM. so resourcefulness & efficiency are key.