212. 单词搜索 II

Java：搜索全部可能

class Solution {
    StringBuilder sb;
    List<String> list;
    Set<String> set;

    private void dfs(int x, int y, int m, int n, char[][] board){
        if (x < 0 || x >= m || y < 0 || y >= n || board[x][y] == '.') {
            return;
        }
        if (sb.length() > 10) {  // 剪枝，不然超时
            return;
        }
        char ch = board[x][y];  
        sb.append(ch);        // 放在set判断之前
        if (set.contains(sb.toString())) {
            list.add(sb.toString());
            set.remove(sb.toString()); // oaa oa oa 题意没告诉不能重复放入ans
        }
        board[x][y] = '.';
        dfs(x + 1, y, m, n, board);
        dfs(x - 1, y, m, n, board);
        dfs(x, y + 1, m, n, board);
        dfs(x, y - 1, m, n, board);
        board[x][y] = ch;
        sb.deleteCharAt(sb.length() - 1);
    }

    public List<String> findWords(char[][] board, String[] words) {
        sb = new StringBuilder();
        list = new ArrayList<>();
        set = new HashSet<>();
        for (String str : words) {
            set.add(str);
        }
        int m = board.length, n = board[0].length;

        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                dfs(i, j, m, n, board);
            }
        }
        return list;
    }
}