| # | Submission time | Handle | Problem | Language | Result | Execution time | Memory |
|---|---|---|---|---|---|---|---|
| 769663 | 2023-06-30T01:33:44 Z | sraeli | Archery (IOI09_archery) | C++ | 0 ms | 0 KB |
#include <iostream>
#include <cassert>
#include <cstring>
#include <utility>
using namespace std;
#define MAX_N 250000
#define MAX_M 1000000000
int n, m;
int ranks[MAX_N * 2];
int rank;
pair<int, int> cache[MAX_N];
bool cached[MAX_N];
int black[3 * MAX_N + 1], grey[3 * MAX_N + 1], white[3 * MAX_N + 1];
/**
* An O(N) simulation of the tournament, given our starting
* position. We also return the number of times that the
* we get bumped from target 1 to target N.
*/
pair<int, int> simulate(int start)
{
// As we might run multiple binary searches, we cache
// the output from this routine for efficiency.
int targ = start >> 1;
if (cached[targ])
return cache[targ];
if (rank == 1)
cache[targ] = make_pair(1, 0);
else if (rank <= n + 1)
{
// We're part of the group of archers that circles
// around after 2*N rounds. To find out where we
// will finish after M rounds, we only consider what
// happens on target 1. After 2*N rounds have taken
// place, we note the next round at which we compete
// on target 1 -- we can then easily compute where
// we will finish, since for each successive round
// we will move to the next target (as we'll be in
// the cycle phase).
memset(black, 0, sizeof(black));
memset(grey, 0, sizeof(grey));
memset(white, 0, sizeof(white));
// Update the initial p values of the archers -- this is the
// minimum number of rounds that it will take an archer to
// reach the first target.
grey[start >> 1] = 1;
for (int i = 1; i < n * 2; i++)
{
int target = (i - 1 + (i > start ? 1 : 0)) >> 1;
if (ranks[i] < rank)
black[target]++;
else
white[target]++;
}
// Work out the ranks of the first two archers on the first target
int archer1, archer2;
if (start < 2)
{
archer1 = ranks[1];
archer2 = rank;
}
else {
archer1 = ranks[1];
archer2 = ranks[2];
}
// And convert the ranks into counts of black, grey and white
// archers.
int s_black = 0, s_grey = 0, s_white = 0;
if (archer1 < rank)
s_black++;
else if (archer1 == rank)
s_grey++;
else
s_white++;
if (archer2 < rank)
s_black++;
else if (archer2 == rank)
s_grey++;
else
s_white++;
int cumulative_black = 0, cumulative_grey = 0, cumulative_white = 0;
int seen = -1;
int bumps = 0;
for (int round = 0; round < 3 * n; round++)
{
// Check if we've seen ourselves on target 1 after
// enough rounds have passed.
if (round >= 2 * n && (s_grey == 1))
{
seen = round;
break;
}
// Determine the colour of the loser of this round
int loser;
if (s_black > 0)
{
if (s_black == 2)
loser = 0;
else if (s_grey == 1)
{
loser = 1;
bumps++;
}
else
loser = 2;
}
else
loser = 2;
// We expect the loser to make it back to the first target n
// rounds from now at best (if they win all of their matches).
int new_p = round + n;
if (new_p <= 3 * n)
{
// We only consider cases below 3n, since that is as far
// as we need to simulate.
if (loser == 0)
{
black[new_p]++;
s_black--;
}
else if (loser == 1)
{
grey[new_p]++;
s_grey--;
}
else
{
white[new_p]++;
s_white--;
}
}
// Now pick an archer to move onto target 1. We add to our
// consideration list all of the archers who we thought would
// make it by the coming round.
cumulative_black += black[round + 1];
cumulative_grey += grey[round + 1];
cumulative_white += white[round + 1];
// Now pick the best archer out of our consideration
// list.
if (cumulative_black > 0)
{
s_black++;
cumulative_black--;
}
else if (cumulative_grey > 0)
{
s_grey++;
cumulative_grey--;
}
else
{
s_white++;
cumulative_white--;
}
}
if (m > seen)
bumps++;
cache[targ] = make_pair(n - ((m - seen + n - 1) % n), bumps);
}
else {
// We're part of the group of weak archers that
// gets stuck on some target after 2*N rounds. We
// just need to work out what that target is.
//
// The idea is that as we simulate the tournament, we can only
// end up with at most one weak archer on any target. So to
// work out where we end up, we "push" the weaker archers around
// the targets. Specifically, we only push ourself (the grey
// archer) and all the archers weaker than us (white archers).
// Keep track of the number of grey and white archers on each
// target.
int white[n], grey[n];
memset(white, 0, sizeof(white));
memset(grey, 0, sizeof(grey));
grey[start >> 1] = 1;
for (int i = 1; i < 2 * n; i++)
if (ranks[i] > rank)
white[(i - 1 + (i > start ? 1 : 0)) >> 1]++;
// Now we push them around. We start at the first target and follow
// the targets that archers would get moved to for losing on each
// target, keeping track of how many archers we are pushing around.
int shift_white = 0, shift_grey = 0;
int bumps = 0;
// It should take 2n rounds for the archers to settle. However, we
// will only pick up some of the archers towards the end of our first
// round, so we run for 3n rounds to accommodate this.
for (int it = 0; it < 3; it++)
{
// Start at the first target
int pos = 0;
do {
// Calculate the total number of archers we have on this
// target, including the ones that we've pushed from the
// previous target.
int cur_white = white[pos] + shift_white;
int cur_grey = grey[pos] + shift_grey;
// Now leave an archer behind and work out how many we
// should push to the next target.
if (cur_white + cur_grey > 1)
{
// More than one grey + white archer, so we must
// leave one behind and push the others.
if (pos > 0)
{
// If this is not the first target, then the
// grey archer advances, if present, and a
// white one stays.
white[pos] = 1;
shift_white = cur_white - 1;
grey[pos] = 0;
if (cur_grey > 0)
shift_grey = cur_grey;
else
shift_grey = 0;
}
else {
// If this is the first target, then the grey
// one stays (if present) and the white ones
// get pushed.
if (cur_grey > 0)
{
grey[pos] = 1;
white[pos] = 0;
shift_grey = cur_grey - 1;
shift_white = cur_white;
}
else
{
grey[pos] = 0;
white[pos] = 1;
shift_grey = 0;
shift_white = cur_white - 1;
}
}
}
else {
// Only one white or grey archer, so either leave
// them behind or push them depending on which target
// we are on.
if (pos > 0)
{
white[pos] = cur_white;
grey[pos] = cur_grey;
shift_white = 0;
shift_grey = 0;
}
else
{
if (cur_grey > 0)
bumps++;
white[pos] = 0;
grey[pos] = 0;
shift_white = cur_white;
shift_grey = cur_grey;
}
}
// Move onto the next position.
if (pos == 0)
pos = n - 1;
else
pos--;
} while (pos > 0);
}
int i;
for (i = 0; i < n; i++)
if (grey[i] > 0)
{
cache[targ] = make_pair(i + 1, bumps);
break;
}
// Sanity check
assert(i < n && grey[i] > 0);
}
cached[targ] = 1;
return cache[targ];
}
/**
* Binary search for the best place to start within a given range
* of starting positions. This assumes that our ending position will
* not wrap as we try different starting positions; see the main
* method below for details of how this is used.
*/
pair<int, int> search(int s, int e)
{
pair<int, int> start, end, mid;
int mi;
start = simulate(s * 2 - 1);
end = simulate(e * 2 - 1);
while ((e - s) > 1)
{
mi = (s + e) >> 1;
mid = simulate(mi * 2 - 1);
if (start.first < mid.first)
{
e = mi;
end = mid;
}
else {
s = mi;
start = mid;
}
}
if (start.first < end.first)
return make_pair(start.first, s);
else
return make_pair(end.first, e);
}
int main()
{
memset(cached, 0, sizeof(cached));
cin >> n >> m;
assert(1 <= n && n <= MAX_N);
assert(2 * n <= m && m <= MAX_M);
// We reduce m to take advantage of the fact that after
// 2n rounds, the archers move around in a cyclical pattern.
m = 2 * n + (m % n);
for (int i = 0; i < n * 2; i++)
cin >> ranks[i];
rank = ranks[0];
// We binary search to find the target to start at. To do this,
// we may require two searches (one to find the point at which
// we wrap around to the next target, and one to then find the
// best target).
int best_start;
pair<int, int> start, end, mid;
int s = 1, e = n, mi;
start = simulate(s * 2 - 1);
end = simulate(e * 2 - 1);
if (start.second > end.second)
{
// There's a wrap. Start by finding the wrapping point.
while ((e - s) > 1)
{
mi = (s + e) >> 1;
mid = simulate(mi * 2 - 1);
if (mid.second > end.second)
{
// We've wrapped, and so have gone too far.
s = mi;
start = mid;
}
else
{
// No wrap yet. Better push it a bit further.
e = mi;
end = mid;
}
}
if (start.second > end.second)
mi = e;
else
mi = s;
pair<int, int> start_side = search(1, mi - 1);
pair<int, int> end_side = search(mi, n);
if (start_side.first < end_side.first)
best_start = start_side.second;
else
best_start = end_side.second;
}
else
{
// No wrap, just binary search for the best place to
// start.
pair<int, int> best = search(1, n);
best_start = best.second;
}
cout << best_start << endl;
return 0;
}