Here I am using 2 arrays to calculate GCD from the starting and from the end. A[i] is the array given for which I am calculating GCD. If I am using the commented part of code to calculate and store GCD's then output I am getting is correct but I am getting TLE error for higher inputs while the statements " backward[i] = gcd(backward[i+1], A[i]); " and " forward[i] = gcd(forward[i-1], A[i]); " are showing correct results without TLE.

Can someone explain what is the difference between the above 2 kinds of statements. why the commented statements are giving TLE ?

'''
int gcd_f = 0, gcd_b = 0;
	for(int i = 1; i <= N; i++){
		  **//gcd_f = gcd(gcd_f, A[i]);
			//forward[i] = gcd_f;**			
			forward[i] = __gcd(forward[i-1], A[i]);
		}
    for(int i = N; i >= 1; i--){		
		  **//gcd_b = gcd(gcd_b, A[i]);
			//backward[i] = gcd_b;**			
			backward[i] = gcd(backward[i+1], A[i]);
    }
	
'''