(Analysis by Spencer Compton)

We start by thinking about the structure of Bessie's journey through time. Since there are only time portals on years that are multiples of 12, and none of Bessie's relatives are born in such a year, to visit some relative Bessie must also visit the preceding year of the Ox and wait 12 years. For example, if Bessie has a relative from 15 years ago, Bessie must visit the year of the Ox 24 years ago and must wait until at least the year of the Ox 12 years ago. In other words, we can think of each year $x$ as belonging to a 12-year cycle $\lfloor \frac{x+11}{12} \rfloor$, so $0$ belongs to cycle $0$, $[1, \dots, 12]$ to cycle $1$, $[13, \dots, 24]$ to cycle $2$, and so on. Meaning, if Bessie has a relative in cycle $x$ then Bessie must spend all 12 years in that cycle.

We must use a jump to go back to the earliest cycle, then with the remaining $K-1$ jumps Bessie can skip over contiguous ranges of unnecessary cycles. It is then optimal to skip over the $K-1$ largest contiguous ranges of unused cycles. One way we can accomplish this is by identifying all the cycles Bessie has relatives in, sorting them, identifying the gaps between adjacent cycles in the sorted list, and sorting those gaps to find the $K-1$ largest. In total, this takes $O(n \log (n) )$ time.

Brian Dean's code:

#include <iostream>
#include <set>
#include <vector>
#include <algorithm>
using namespace std;

set<int> blocks;
vector<int> gaps;

int main(void)
{
int N, K, years_ago, answer, last = 0;
cin >> N >> K;
for (int i=0; i<N; i++) { cin >> years_ago; blocks.insert ((years_ago+11)/12); }