Otherwise good solution

?? LOL

Doesn't work. Fatal flaw. Otherwise good solution.

Will fail if array[0] == array[1] and the array is sorted ascending (array[i]<=array[i+1]).
Otherwise good solution.

Sounds like you're declaring a variable but never assigning it a value before you are trying to read its value.
int i; // i is not initialized (but declared)
j = i + 1; // error

int i = 0; // i is initialized and declared
j = i + 1; // j = 1