Leetcode694-numberOfDistinctIslands

Description

Given a non-empty 2D array grid of 0’s and 1’s, an island is a group of 1’s (representing land) connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.

Count the number of distinct islands. An island is considered to be the same as another if and only if one island can be translated (and not rotated or reflected) to equal the other.

Example

Example 1:

Given the above grid map, return 1.

Example 2:

Given the above grid map, return 3.

Notice that:

1
2

11
1

and

1
2

 1
11

are considered different island shapes, because we do not consider reflection / rotation.

Note: The length of each dimension in the given grid does not exceed 50.

Solution

class Solution {
    public int numDistinctIslands(int[][] grid) {
        if (grid == null || grid.length == 0 || grid[0].length == 0) return 0;
        HashSet<String> set = new HashSet<>();
        for (int i = 0; i < grid.length; i++){
            for (int j = 0; j < grid[0].length; j++){
                if (grid[i][j] != 0){
                    StringBuilder cur= new StringBuilder();
                    helper(grid, i, j, cur, "o");
                    set.add(cur.toString());
                }
            }
        }
        return set.size();
    }
    
    private int[][] direction = new int[][]{{1, 0},{-1, 0},{0, 1},{0, -1}};
    private String[] symbols = new String[]{"d", "u", "r", "l"};
    private void helper(int[][] grid, int x, int y, StringBuilder cur, String dir){
        grid[x][y] = 0;
        cur.append(dir);
        for (int i = 0; i < direction.length; i++){
            int nx = x + direction[i][0];
            int ny = y + direction[i][1];
            if (nx >= 0 && nx < grid.length && ny >= 0 && ny < grid[0].length && grid[nx][ny] == 1){
                helper(grid, nx, ny, cur, symbols[i]);
            }
        }
        cur.append("b");
    }
}