C++の練習を兼ねて, AtCoder Grand Contest 014 の 問題C (Closed Rooms) を解いてみた.
■感想.
1. 問題Cは, 方針が見えなかったので, 解説を参考に実装して, AC版に到達出来た.
2. 幅優先探索の復習が出来たので, 非常に良かったと思う.
3. 引き続き, 時間を見つけて, 過去問の学習を進めていきたいと思う.
本家のサイト AtCoder Grand Contest 014 解説 を ご覧下さい.
■C++版プログラム(問題C／AC版).
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// 解き直し. // https://img.atcoder.jp/agc014/editorial.pdf // C++(GCC 9.2.1) #include <bits/stdc++.h> using namespace std; using vs = vector<string>; #define repex(i, a, b, c) for(int i = a; i < b; i += c) #define repx(i, a, b) repex(i, a, b, 1) #define rep(i, n) repx(i, 0, n) #define repr(i, a, b) for(int i = a; i >= b; i--) #define pb push_back constexpr int dx[] = {-1, 0, 1, 0}, dy[] = {0, -1, 0, 1}; int MAX = 808 * 808; int main(){ // 1. 入力情報. int H, W, K, sh = -1, sw = -1; scanf("%d %d %d", &H, &W, &K); vs G; rep(i, H){ char c[808]; scanf("%s", c); string s(c); rep(j, W){ if(s[j] == 'S'){ sh = i; sw = j; s[j] = '.'; } } G.pb(s); } // 2. bfs. // https://ja.wikipedia.org/wiki/幅優先探索 auto bfs = [&](vs &G, int s, int* d) -> void { // 空のキュー. queue<int> q; // 探索地点 s をキュー q に追加. q.push(s); while(!q.empty()){ // キューから取り出す. int v = q.front(); q.pop(); // 取り出した要素を処理. // x: 列方向, y: 行方向 で考える. int nx, ny, n; int cx = v % W , cy = v / W; rep(i, 4){ nx = cx + dx[i]; ny = cy + dy[i]; n = nx + ny * W; // 訪問不可能なマス であれば, 処理をスキップ. if(s == n || nx < 0 || nx >= W || ny < 0 || ny >= H) continue; // 訪問先のマスで, 訪問可能 かつ 未訪問 であれば, 最短距離 を 設定. if(G[ny][nx] == '.' && !d[n]) d[n] = d[v] + 1, q.push(n); } } }; // 3. 今いる部屋から, 各部屋までの距離を計算. int d[MAX], s = sw + sh * W; rep(i, MAX) d[i] = 0; bfs(G, s, d); // 4. 最小回数は？ int magic = 2020202020; rep(i, H * W){ // 閉じられている部屋は, Skip. if(d[i] > K) continue; // 今いる部屋以外で, 距離 0 の 部屋を Skip. if(!d[i] && (i != s)) continue; // 到達先. int t = 2020202020, cx = i % W , cy = i / W; // 1行目, 1列目, H行目, W列目. t = min({t, cy, cx, H - 1 - cy, W - 1 - cx}); // 最小値更新. magic = min(magic, 1 + (t + K - 1) / K); } // 5. 出力. printf("%d\n", magic); return 0; } |
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■参照サイト
AtCoder Grand Contest 014