0 th , 1 st and 2 nd number of the series are 1
int pPrevPrev = 1 , pPrev = 1 , pCurr = 1 , pNext = 1 ; for ( int i = 3 ; i <= n ; i ++ ) { pNext = pPrevPrev + pPrev ; pPrevPrev = pPrev ; pPrev = pCurr ; pCurr = pNext ; } return pNext ; }