算法奇妙屋(十五)-BFS解决边权为1的单源最短路径问题

    int[] dx = new int[]{-1, 1, 0, 0};
    int[] dy = new int[]{0, 0, -1, 1};
    public int nearestExit(char[][] maze, int[] entrance) {
        int m = maze.length;
        int n = maze[0].length;
        boolean[][] vis = new boolean[m][n];
        vis[entrance[0]][entrance[1]] = true;
        Queue<Integer> q = new LinkedList<>();
        q.offer(entrance[0]);
        q.offer(entrance[1]);
        int count = 0;
        while (!q.isEmpty()) {
            int size = q.size();
            count++;
            while (size > 0) {
                int a = q.poll();
                int b = q.poll();
                for (int i = 0; i < 4; i++) {
                    int x = dx[i] + a;
                    int y = dy[i] + b;
                    if (x >= 0 && x < m && y >= 0 && y < n && maze[x][y] == '.' && !vis[x][y]) {
                        if (x == 0 || x == m - 1 || y == 0 || y == n - 1) {
                            return count;
                        }
                        q.offer(x);
                        q.offer(y);
                        vis[x][y] = true;
                    }
                }
                size -= 2;
            }
        }
        return -1;
    }

    char[] chs = { 'A', 'C', 'G', 'T' };
    public  int minMutation(String startGene, String endGene, String[] bank) {
        Set<String> hash = new HashSet<>();
        Set<String> vis = new HashSet<>();
        for (String s : bank) {
            vis.add(s);
        }
        int n = startGene.length();
        Queue<String> q = new LinkedList<>();
        q.offer(startGene);
        hash.add(startGene);
        int ret = 0;
        while (!q.isEmpty()) {
            int size = q.size();
            ret++;
            while (size > 0) {
                String s = q.poll();
                for (int i = 0; i < n; i++) {
                    for (int j = 0; j < 4; j++) {
                        StringBuilder stringBuilder = new StringBuilder();
                        stringBuilder.append(s.substring(0, i));
                        stringBuilder.append(chs[j]);
                        if (i < n - 1) {
                            stringBuilder.append(s.substring(i + 1, n));
                        }
                        String tmp = stringBuilder.toString();
                        if (!hash.contains(tmp) && vis.contains(tmp)) {
                            if (tmp.equals(endGene)) {
                                return ret;
                            }
                            q.offer(tmp);
                        }
                        hash.add(tmp);
                    }
                }
                size--;
            }
        }
        return -1;
    }

    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        Set<String> hash = new HashSet<>();
        Set<String> vis = new HashSet<>();
        for (String s : wordList) {
            vis.add(s);
        }
        if (!vis.contains(endWord)) {
            return 0;
        }
        Queue<String> q = new LinkedList<>();
        hash.add(beginWord);
        q.offer(beginWord);
        int ret = 1;
        while (!q.isEmpty()) {
            int size = q.size();
            ret++;
            while (size-- > 0) {
                String ss = q.poll();
                for (int i = 0; i < ss.length(); i++) {
                    char[] ch = ss.toCharArray();
                    for (char j = 'a'; j <= 'z'; j++) {
                        ch[i] = j;
                        String s = new String(ch);
                        if (!hash.contains(s) && vis.contains(s)) {
                            if (s.equals(endWord)) {
                                return ret;
                            }
                            q.offer(s);
                        }
                        hash.add(s);
                    }
                }
            }
        }
        return 0;
    }

    int[] dx = { 0, 0, 1, -1};
    int[] dy = { 1, -1, 0, 0};
    int m, n;
    public int cutOffTree(List<List<Integer>> f) {
        m = f.size();
        n = f.get(0).size();
        List<int[]> init = new ArrayList<>();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (f.get(i).get(j) > 1) {
                    init.add(new int[] { i, j });
                }
            }
        }
        // 排序
        Collections.sort(init, (a, b) -> {
            return f.get(a[0]).get(a[1]) - f.get(b[0]).get(b[1]);
        });
        int ret = 0;
        int bx = 0;
        int by = 0;
        for (int[] tree : init) {
            int ex = tree[0];
            int ey = tree[1];
            int step = maze(f, bx, by, ex, ey);
            if (step == -1) {
                return -1;
            }
            ret += step;
            bx = ex;
            by = ey;
        }
        return ret;
    }

    int maze(List<List<Integer>> f, int bx, int by, int ex, int ey) {
        if (bx == ex && by == ey) {
            return 0;
        }
        Queue<int[]> q = new LinkedList<>();
        boolean[][] vis = new boolean[m][n];
        vis[bx][by] = true;
        q.offer(new int[]{bx, by});
        int ret = 0;
        while (!q.isEmpty()) {
            int size = q.size();
            ret++;
            while (size-- > 0) {
                int[] tmp = q.poll();
                int a = tmp[0];
                int b = tmp[1];
                for (int i = 0; i < 4; i++) {
                    int x = a + dx[i];
                    int y = b + dy[i];
                    if (x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] && f.get(x).get(y) != 0 ) {
                        if (x == ex && y == ey) {
                            return ret;
                        }
                        q.offer(new int[]{x,y});
                        vis[x][y] = true;
                    }
                }
            }
        }
        return -1;
    }