Can someone please explain to me what the logic behind the solution to this problem is:
"A Simple Equation" - http://www.codechef.com/problems/EQUATION
I looked at a few solutions that produced the correct answer and I still cannot figure out why it works.
For example:
#include <stdio.h>
long long f(long long n)
{
long long r;
if (n<0)
return 0;
r = ((n + 1)*(n + 2)*(n + 3));
return r / 6;
}
main()
{
int t;
long long n, a, b, c, ct;
scanf("%d", &t);
while (t--){
scanf("%lld%lld%lld%lld", &n, &a, &b, &c);
ct = f(n) - f(n - a - 1) - f(n - b - 1) - f(n - c - 1) + f(n - a - b - 2) + f(n - b - c - 2) + f(n - a - c - 2) - f(n - a - b - c - 3);
printf("%lld\n", ct);
}
return 0;
}