Program #34 (U3): Recursion — Sum from 1 to N
Task: Read N and compute 1 + 2 + … + N recursively with a clear base case. Print the sum.
#include <stdio.h>
long long sum_rec(long long n){ return (n<=0)?0:n+sum_rec(n-1); }
int main(void){
long long n;
printf("Enter N (>=0): ");
if(scanf("%lld", &n)!=1 || n<0){ printf("Invalid N.\n"); return 0; }
printf("Sum = %lld\n", sum_rec(n));
return 0;
}