【BFS最短路】迷宫中离入口最近的出口 / 最小基因变化 / 单词接龙 / 为高尔夫比赛砍树

class Solution {
public:
    int nearestExit(vector<vector<char>>& maze, vector<int>& entrance) {
        int dx[4] = {1, -1, 0, 0}, dy[4] = {0, 0, 1, -1};
        bool used[101][101] = {};
        int m = maze.size(), n = maze[0].size(), step = 0;
        queue<pair<int, int>> q;
        int row = entrance[0], col = entrance[1];
        q.push({row, col});
        used[row][col] = true;
        while (q.size())
        {
            step++; // 向外扩展一层
            int sz = q.size();
            while (sz--) 
            {
                auto [a, b] = q.front();
                q.pop();
                for (int i = 0; i < 4; i++)
                {
                    int x = a + dx[i], y = b + dy[i];
                    if (x >= 0 && x < m && y >= 0 && y < n && maze[x][y] == '.' 
                        && !used[x][y])
                    {
                        if (x == 0 || x == m - 1 || y == 0 || y == n - 1) return step;
                        used[x][y] = true;
                        q.push({x, y});
                    }
                }
            }
        }
        return -1;
    }
};

class Solution {
public:
    int minMutation(string startGene, string endGene, vector<string>& bank) {
        unordered_set<string> hash(bank.begin(), bank.end());
        unordered_set<string> used;
        const string str = "ACGT";

        if (startGene == endGene) return 0;
        if (!hash.count(endGene)) return -1;

        queue<string> q;
        q.push(startGene);
        used.insert(startGene);
        int step = 0;
        while (q.size())
        {
            step++;
            int sz = q.size();
            while (sz--)
            {
                string s = q.front();
                q.pop();
                for (int i = 0; i < 8; i++)
                {
                    string tmp = s;
                    for (int j = 0; j < 4; j++)
                    {
                        tmp[i] = str[j];
                        if (tmp == endGene) return step;
                        if (hash.count(tmp) && !used.count(tmp))
                        {
                            used.insert(tmp);
                            q.push(tmp);
                        }
                    }
                }
            }
        }
        return -1;
    }
};

class Solution {
public:
    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
        unordered_set<string> hash(wordList.begin(), wordList.end());
        unordered_set<string> used;
        queue<string> q;
        if (!hash.count(endWord)) return 0;
        q.push(beginWord);
        used.insert(beginWord);
        int ret = 1; // 最初beginWord也算一个单词
        while (q.size())
        {
            ret++;
            int sz = q.size();
            while (sz--)
            {
                string str = q.front();
                q.pop();
                for (int i = 0; i < beginWord.size(); i++)
                {
                    string tmp(str);
                    for (char ch = 'a'; ch <= 'z'; ch++)
                    {
                        tmp[i] = ch;
                        if (tmp == endWord) return ret;
                        if (hash.count(tmp) && !used.count(tmp))
                        {
                            used.insert(tmp);
                            q.push(tmp);
                        }
                    }
                }
            }
        }
        return 0;
    }
};

class Solution {
    int dx[4] = {1, -1, 0, 0}, dy[4] = {0, 0, 1, -1};
    int m, n, ret = 0;
public:
    int cutOffTree(vector<vector<int>>& forest) {
        m = forest.size(), n = forest[0].size();
        map<int, pair<int, int>> hash;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (forest[i][j] > 1)
                    hash[forest[i][j]] = {i, j};
        int bx = 0, by = 0;
        for (auto &[a, b] : hash)
        {
            int step = bfs(forest, bx, by, b.first, b.second);
            if (step == -1) return -1;
            ret += step;
            bx = b.first;
            by = b.second;
        }
        return ret;
    }
    int bfs(vector<vector<int>>& forest, int bx, int by, int ex, int ey)
    {
        if (bx == ex && by == ey) return 0;
        int ret = 0;
        queue<pair<int, int>> q;
        bool used[51][51] = {};
        q.push({bx, by});
        used[bx][by] = true;
        while (q.size())
        {
            ret++;
            int sz = q.size();
            while (sz--)
            {
                auto [a, b] = q.front();
                q.pop();
                for (int i = 0; i < 4; i++)
                {
                    int x = a + dx[i], y = b + dy[i];
                    if (x == ex && y == ey) return ret;
                    if (x >= 0 && x < m && y >= 0 && y < n && forest[x][y] && !used[x][y])
                    {
                        used[x][y] = true;
                        q.push({x, y});
                    }
                }
            }
        }
        return -1;
    }
};

class Solution {
    int dx[4] = {1, -1, 0, 0}, dy[4] = {0, 0, 1, -1};
    int m, n, ret = 0;
public:
    int cutOffTree(vector<vector<int>>& f) {
        m = f.size(), n = f[0].size();
        vector<pair<int, int>> trees;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (f[i][j] > 1) 
                    trees.push_back({i, j});
        sort(trees.begin(), trees.end(), 
            [&f](const pair<int, int>& p1, const pair<int, int>& p2)
            { return f[p1.first][p1.second] < f[p2.first][p2.second];});
        
        int bx = 0, by = 0;
        for (auto [a, b] : trees)
        {
            int step = bfs(f, bx, by, a, b);
            if (step == -1) return -1;
            ret += step;
            bx = a, by = b;
        }
        return ret;
    }
    int bfs(vector<vector<int>>& f, int bx, int by, int ex, int ey)
    {
        // 处理边界情况
        if (bx == ex && by == ey) return 0;
        queue<pair<int, int>> q;
        bool used[51][51] = {};
        q.push({bx, by});
        used[bx][by] = true;
        int ret = 0;
        while (q.size())
        {
            ret++;
            int sz = q.size();
            while (sz--)
            {
                auto [a, b] = q.front();
                q.pop();
                for (int i = 0; i < 4; i++)
                {
                    int x = a + dx[i], y = b + dy[i];
                    if (x == ex && y == ey) return ret;
                    if (x < 0 || x >= m || y < 0 || y >= n || !f[x][y] || used[x][y])
                        continue;
                    used[x][y] = true;
                    q.push({x, y});
                }
            }
        }
        return -1;
    }
};