Submission #1137326

#	Time	Username	Problem	Language	Result	Execution time	Memory
1137326		henrlly	Tracks in the Snow (BOI13_tracks)	C++20	100 / 100	455 ms	126996 KiB

tracks

// time-limit: 3000
#include <bits/stdc++.h>
#include <cstring>
#include <vector>

using namespace std;

#define ll long long

using vll = vector<ll>;
using vvll = vector<vector<ll> >;
using mpll = map<pair<ll, ll>, ll>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vvi = vector<vector<int> >;
using pi = pair<int, int>;

void setIO(string name = "") {
    if (name.size()) {
        freopen((name + ".in").c_str(), "r", stdin);
        freopen((name + ".out").c_str(), "w", stdout);
    }
}

void solve() {
    return;
}

int weight[4001][4001];
char m[4001][4001];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    // setIO("test");

    // for optimal solution, each color crosses meadow alternatingly
    // if same color cross >1 in a row, can just go back from end point for all the subsequent paths
    // construct tree that connects each continguous area of one color to all adjacent blobs of area of the other color
    // depth of that tree is min paths (the answer)
    //
    // alternatively, directly find the node whose shortest path from 0,0 is furthest (in terms of sum of
    // weight of edges in the path to that node), weight of adjacent cell of same color = 0, diff color = 1
    // the length of the furthest shortest-path is the ans
    int h, w;
    cin >> h >> w;
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            cin >> m[i][j];
        }
    }

    memset(weight, -1, sizeof(weight));
    weight[0][0] = 0;
    int di[4] = {0,0,-1,1};
    int dj[4] = {-1,1,0,0};

    int res = 0;
    deque<pair<int,int>> dq;
    dq.push_back({0,0});
    while (!dq.empty()) {
        auto p = dq.front();
        dq.pop_front();
        int ww = weight[p.first][p.second];
        res = max(res, ww);
        for (int i = 0; i < 4; i++) {
            int ii = p.first + di[i];
            int jj = p.second + dj[i];
            if (ii < 0 || jj < 0 || ii >= h || jj >= w) continue;
            if (m[ii][jj] == '.') continue;
            if (weight[ii][jj] != -1) continue;
            if (m[ii][jj] == m[p.first][p.second] ) {
                weight[ii][jj] = ww;
                dq.push_front({ii, jj});
            } else {
                weight[ii][jj] = ww + 1;
                dq.push_back({ii, jj});
            }
        }
    }

    cout << res + 1 << endl;

    return 0;
}

Compilation message (stderr)

tracks.cpp: In function 'void setIO(std::string)':
tracks.cpp:20:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   20 |         freopen((name + ".in").c_str(), "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tracks.cpp:21:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   21 |         freopen((name + ".out").c_str(), "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 30 / 30
• Subtask 2: 70 / 70