79. 单词搜索

79. 单词搜索

https://leetcode-cn.com/problems/word-search/ 

/// 79. Word Search
/// Source : https://leetcode.com/problems/word-search/description/
///
/// 回溯法
/// 时间复杂度: O(m*n*m*n)
/// 空间复杂度: O(m*n)
public class Solution {
    private int d[][] = {
    {-1, 0}, {0, 1}, {1, 0}, {0, -1}};
    private int m, n;
    private boolean[][] visited;

    public boolean exist(char[][] board, String word) {
        //if (board == null || word == null){
        //    throw new IllegalArgumentException("board or word can not be null!");
        //}
        m = board.length;
        //if (m == 0){
        //    throw new IllegalArgumentException("board can not be empty.");
        //}
        n = board[0].length;
        //if (n == 0){
        //    throw new IllegalArgumentException("board can not be empty.");
        //}
        visited = new boolean[m][n];
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (searchWord(board, word, 0, i, j)) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean inArea(int x, int y) {
        return x >= 0 && x < m && y >= 0 && y < n;
    }

    // 从board[startx][starty]开始, 寻找word[index...word.size())
    private boolean searchWord(char[][] board, String word, int index, int startx, int starty) {
        //assert(inArea(startx,starty));
        if (index == word.length() - 1){
            return board[startx][starty] == word.charAt(index);
        }
        if (board[startx][starty] == word.charAt(index)) {
            visited[startx][starty] = true;
            // 从startx, starty出发,向四个方向寻
            for (int i = 0; i < 4; i++) {
                int newx = startx + d[i][0];
                int newy = starty + d[i][1];
                if (inArea(newx, newy) && !visited[newx][newy] && searchWord(board, word, index + 1, newx, newy)) {
                    return true;
                }
            }
            visited[startx][starty] = false;
        }
        return false;
    }
    //public static void main(String args[]) {
    //    char[][] b1 = {
    {'A', 'B', 'C', 'E'},
    //            {'S', 'F', 'C', 'S'},
    //            {'A', 'D', 'E', 'E'}};
    //
    //    String words[] = {"ABCCED", "SEE", "ABCB"};
    //    for (int i = 0; i < words.length; i++)
    //        if ((new Solution()).exist(b1, words[i]))
    //            System.out.println("found " + words[i]);
    //        else
    //            System.out.println("can not found " + words[i]);
    //
    //    // ---
    //
    //    char[][] b2 = {
    {'A'}};
    //    if ((new Solution()).exist(b2, "AB"))
    //        System.out.println("found AB");
    //    else
    //        System.out.println("can not found AB");
    //}
}

class Solution {

    //四个方向
    //上 右 下 左
    private int[][] dir = {
    {-1, 0}, {0, 1}, {1, 0}, {0, -1}};

    private boolean[][] visited;
    //行数
    private Integer m;
    //列数
    private Integer n;

    public boolean exist(char[][] board, String word) {
        m = board.length;
        n = board[0].length;
        visited = new boolean[m][n];

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (dfs(i, j, 0, board, word)){
                    return true;
                }
            }
        }
        return false;

    }

    private boolean dfs(int x, int y, int index, char[][] board, String word) {

        if (index == word.length() - 1) {
            return word.charAt(index) == board[x][y];
        }

        if (board[x][y] == word.charAt(index)) {
            visited[x][y] = true;
            for (int i = 0; i < 4; i++) {
                int disX = x + dir[i][0];
                int disY = y + dir[i][1];
                if (areYouOk(disX, disY) && visited[disX][disY] == false) {
                    if (dfs(disX, disY, index + 1, board, word)){
                        return true;
                    }
                    //return dfs(disX, disY, index + 1, board, word);
                }
            }
            visited[x][y] = false;
        }
        return false;
    }

    /**
     * 返回（x,y）是否越界
     * @param x 行
     * @param y 列
     */
    private boolean areYouOk(int x, int y) {
        return x >= 0 && x < m && y >= 0 && y < n;
    }

    //public static void main(String[] args) {
    //    char arr[][] = {
    {'a','a'}};
    //    String str = "aaa";
    //    //false
    //    System.out.println(new Solution().exist(arr, str));
    //}

}