694. Number of Distinct Islands

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 1:

11000
11000
00011
00011
Given the above grid map, return 1.

Example 2:

11011
10000
00001
11011
Given the above grid map, return 3.

Notice that:
11
1
and
 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.




https://leetcode.com/problems/number-of-distinct-islands/discuss/108475/Java-very-Elegant-and-concise-DFS-Solution(Beats-100)


// correct 
class Solution {
    private static final int[][] dirs = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
    public int numDistinctIslands(int[][] grid) {
        
        // convert shape into a string , and add it to a set, if a set contains this shape 
        // then counter doesn't change 
        HashSet<String> set = new HashSet<>();
        int res = 0;
        for(int i = 0; i < grid.length; i++){
            for(int j = 0; j < grid[0].length; j++){
                if(grid[i][j] == 1){
                    StringBuilder sb = new StringBuilder();
                    dfs(grid, i, j, 0, 0, sb);
                    String s = sb.toString();
                    if(!set.contains(s)){
                        set.add(s);
                        res++;
                    }
                }
            }
        }
        return res;   
    }
    private void dfs(int[][] grid, int i, int j, int x, int y, StringBuilder sb){
        grid[i][j] = 0;
        sb.append(x + "" + y); // "" ? 
        for(int[] dir : dirs){
            int row = i + dir[0];
            int col = j + dir[1];
            // check if valid boundary and if its 0 
            if(row < 0 || col < 0 || row >= grid.length || col >= grid[0].length || grid[row][col] == 0) continue;
            dfs(grid, row, col, x + dir[0], y + dir[1], sb);
        }   
    }
}


// wrong answer, can pass 434/750 cases 





Notes: 
Set.equals(element)


Element can only be primitive type, not object, because object compares addresses 

If you want it to compare object, you have to override equals and hashcode 





    private static final int[][] dirs = {{}, {}, {}, {}};

Can’t write new int[][] {} , If you put private static final