Number of indexes with equal elements in given range

Given N numbers and Q queries, every query consists of L and R, task is to find the number of such integers i (L<=i<R) such that A_i=A_i+1. Consider 0-based indexing. 

Examples : 

Input : A = [1, 2, 2, 2, 3, 3, 4, 4, 4] 
 Q = 2 
 L = 1 R = 8 
 L = 0 R = 4 
Output : 5 
 2
Explanation: We have 5 index i which has 
Ai=Ai+1 in range [1, 8). We have 
2 indexes i which have Ai=Ai+1
in range [0, 4). 

Input :A = [3, 3, 4, 4] 
 Q = 2
 L = 0 R = 3
 L = 2 R = 3 
Output : 2 
 1

A naive approach is to traverse from L to R (Exclusive R) and count the number of index i which satisfies the condition A_i=A_i+1 for every query separately. 

Below is the implementation of the naive approach : 

5
2

Time Complexity : O(R - L) for every query 
Auxiliary Space: O(1)

Efficient approach: We can answer queries in O(1) time. The idea is to precompute a prefix array prefixans such that prefixans[i] stores the total count of the index from 0 to i that obeys the given condition. prefixans[R-1]-prefix[L-1] returns the total count of an index in the range [L, r) for every query. 

Below is the implementation of the efficient approach : 

5
2

Time complexity: O(n)
Auxiliary Space: O(n)