<DFS & BFS> 130 127

130. Surrounded Regions

BFS： 把所有边界上的O先换成A（标记），再深度遍历周围的点。

最后把O换成X（表示不符合要求），所有的A换回O

class Solution {
    public void solve(char[][] board) {
        if(board.length == 0 || board[0].length == 0) return;
        int m = board.length;
        int n = board[0].length;

        for(int i = 0; i < m; i++){
            dfs(board, i, 0);
            dfs(board, i, n - 1);
        }

        for(int j = 1; j < n - 1; j++){
            dfs(board, 0, j);
            dfs(board, m - 1, j);
        }

        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(board[i][j] == 'O') board[i][j] = 'X';
                if(board[i][j] == 'A') board[i][j] = 'O';
            }
        }
    }

    private void dfs(char[][] board, int i, int j){
        if(i < 0 || i >= board.length || j < 0 || j >= board[0].length) return;
        if(board[i][j] == 'X' || board[i][j] == 'A') return;
        board[i][j] = 'A';
        dfs(board, i - 1, j);
        dfs(board, i + 1, j);
        dfs(board, i, j - 1);
        dfs(board, i, j + 1);
    }
}

127. Word Ladder

BFS：queue，使用BFS时必须用到的队列，Set用于放置wordList。按当前

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        Set<String> set = new HashSet<>(wordList);
        Queue<String> queue = new LinkedList<>();
        queue.add(beginWord);
        int level = 0;

        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0; i < size; i++){
                String cur = queue.remove();
                char[] wordUnit = cur.toCharArray();
                if(cur.equals(endWord)) return level + 1;
                for(int j = 0; j < cur.length(); j++){
                    char temp = wordUnit[j];
                    for(char c = 'a'; c <= 'z'; c++){
                        wordUnit[j] = c;
                        String s = new String(wordUnit);
                        if(set.contains(s)){
                            queue.add(s);
                            set.remove(s);
                        }
                    }
                    wordUnit[j] = temp;
                }
            }
            level++;
        }
        return 0;
    }
}