Sumitomo Mitsui Trust Bank Programming Contest 2019 Task F. Interval Running

Link.

There is a nice approach to this problem that involves some physical insight.
In the following we'll refer to Takahashi as A and to Aoki as B.

Without loss of generality, assume \(A_1 > B_1\). Consider A's relative motion with respect to B, i.e. imagine B is always at rest. The relative velocity of A is \(A_1 - B_1\) for the first \(T_1\) minutes and \(A_2 - B_2\) for the subsequent \(T_2\) minutes.

Let \(S = (A_1 - B_1) T_1 + (A_2 - B_2) T_2\), we have

1. if \(S > 0\), A and B never meet,
2. if \(S = 0\), they meet infinitely many times,
3. if \(S < 0\), they meet some finite number of times.

In case \(S < 0\), it's not hard to figure out the number of times A and B meet.

code

 int main() {
    ll t1, t2;
    ll a1, a2, b1, b2;
    scan(t1, t2, a1, a2, b1, b2);
b1 -= a1;

b2 -= a2;

if (b1 < 0) {

b1 = -b1;

b2 = -b2;

}

ll s = b1 * t1 + b2 * t2;

if (s == 0) {

println("infinity");

} else  if (s > 0) {

println(0);

} else {

s = -s;

ll k = b1 * t1 / s;

ll ans = 2 * k;

if (b1 * t1 % s == 0) {

++ans;

} else {

ans += 2;

}

println(ans - 1); // -1 because we do not count the start of the run as a meet

}

return 0;

}