【算法刷题指南】BFS-解决最短路问题

class Solution {
    int dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};
    typedef pair<int, int> PII;
    bool vis[105][105];

public:
    int nearestExit(vector<vector<char>>& maze, vector<int>& entrance) {
        int m = maze.size(), n = maze[0].size();
        queue<PII> q;
        q.push({entrance[0], entrance[1]});
        vis[entrance[0]][entrance[1]] = 1;
        int cnt = 0;
        while (q.size()) {
            cnt++;
            int sz=q.size();
            for (int i = 0; i < sz; i++) {
                auto [a, b] = q.front();
                q.pop();
                for (int j = 0; j < 4; j++) {
                    int x = a + dx[j], y = b + dy[j];
                    if (x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] &&
                            maze[x][y] == '.') {
                        if (x == 0 || x == m - 1 || y == 0 || y == n - 1) {
                            return cnt;
                        }
                        q.push({x, y});
                        vis[x][y] = 1;
                    }
                }
            }
        }
        return -1;
    }
};

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

        queue<string> q;
        q.push(startGene);
        vis.insert(startGene);

        if(startGene==endGene) return 0;
        if(!hash.count(endGene)) return -1;
        int ans=0;
        while(q.size())
        {
            ans++;
            int sz=q.size();
            while(sz--)
            {
                string t=q.front();
                q.pop();
                for(int i=0;i<8;i++)
                {
                    string tmp=t;
                    for(int j=0;j<4;j++)
                    {
                        tmp[i]=change[j];
                        if(hash.count(tmp)&&!vis.count(tmp))
                        {
                            if(tmp==endGene) return ans;
                            q.push(tmp);
                            vis.insert(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> vis;
        if(!hash.count(endWord)) return 0;

        queue<string> q;
        q.push(beginWord);
        vis.insert(beginWord);
        int ans=1;
        while(q.size())
        {
            ans++;
            int sz=q.size();
            while(sz--)
            {
                string t=q.front();
                q.pop();
                for(int i=0;i<t.size();i++)
                {
                    string tmp=t;
                    for(char ch='a';ch<='z';ch++)
                    {
                        tmp[i]=ch;
                        if(hash.count(tmp)&&!vis.count(tmp))
                        {
                            if(tmp==endWord) return ans;
                            q.push(tmp);
                            vis.insert(tmp);
                        }
                    }
                }
            }
        } 
        return 0;
    }
};

class Solution {
    typedef pair<int, int> PII;
    int m, n;
    int dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};
    bool vis[55][55];

public:
    int bfs(vector<vector<int>>& forest, int bx, int by, int ex, int ey) {
        if (bx == ex && by == ey)
            return 0;

        queue<PII> q;
        q.push({bx, by});
        vis[bx][by] = 1;
        memset(vis, 0, sizeof(vis));
        int step = 0;
        while (q.size()) 
        {
            step++;
            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 && !vis[x][y] &&
                        forest[x][y] != 0) {
                        if (x == ex && y == ey)
                            return step;
                        q.push({x, y});
                        vis[x][y] = 1;
                    }
                }
            }
        }
        return -1;
    }

    int cutOffTree(vector<vector<int>>& forest) 
    {
        m = forest.size(), n = forest[0].size();
        vector<PII> tree;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                if (forest[i][j] > 1)
                    tree.push_back({i, j});

        sort(tree.begin(), tree.end(), [&](const PII& p1, const PII& p2) {
            return forest[p1.first][p1.second] < forest[p2.first][p2.second];
        });

        int bx = 0, by = 0, ret = 0;
        for (auto& [a, b] : tree)
        {
            int step = bfs(forest, bx, by, a, b);
            if (step == -1)
                return -1;
            ret += step;
            bx = a, by = b;
        }
        return ret;
    }
};