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695. Max Area of Island

You are given an m x n binary matrix grid. 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.

The area of an island is the number of cells with a value 1 in the island.

Return the maximum area of an island in grid. If there is no island, return 0.

Example 1:

img

Input: grid = [[0,0,1,0,0,0,0,1,0,0,0,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,1,1,0,1,0,0,0,0,0,0,0,0],[0,1,0,0,1,1,0,0,1,0,1,0,0],[0,1,0,0,1,1,0,0,1,1,1,0,0],[0,0,0,0,0,0,0,0,0,0,1,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,0,0,0,0,0,0,1,1,0,0,0,0]]
Output: 6
Explanation: The answer is not 11, because the island must be connected 4-directionally.

Example 2:

Input: grid = [[0,0,0,0,0,0,0,0]]
Output: 0

Constraints:

  • m == grid.length
  • n == grid[i].length
  • 1 <= m, n <= 50
  • grid[i][j] is either 0 or 1.

Solution:

class Solution {
    int count;
    public int maxAreaOfIsland(int[][] grid) {
        int result = 0;

        for (int i = 0; i < grid.length; i++){
            for (int j = 0; j < grid[0].length; j++){
                if (grid[i][j] == 1){
                    count = 0;
                    helper(grid, i, j);
                    result = Math.max(count, result);
                }
            }
        }

        return result;
    }

    private void helper(int[][] grid, int x, int y){
        if (x < 0 || x >= grid.length || y < 0 || y >= grid[0].length || grid[x][y] == 0){
            return;
        }

        count++;
        grid[x][y] = 0;

        helper(grid, x + 1, y);
        helper(grid, x - 1, y);
        helper(grid, x, y+ 1);
        helper(grid, x, y-1);
    }
}
// TC: O(n^2)
// SC: O(n^2)

class Solution {
    int[][] grid;
    boolean[][] seen;

    public int area(int r, int c) {
        if (r < 0 || r >= grid.length || c < 0 || c >= grid[0].length ||
                seen[r][c] || grid[r][c] == 0)
            return 0;
        seen[r][c] = true;
        return (1 + area(r+1, c) + area(r-1, c)
                  + area(r, c-1) + area(r, c+1));
    }

    public int maxAreaOfIsland(int[][] grid) {
        this.grid = grid;
        seen = new boolean[grid.length][grid[0].length];
        int ans = 0;
        for (int r = 0; r < grid.length; r++) {
            for (int c = 0; c < grid[0].length; c++) {
                ans = Math.max(ans, area(r, c));
            }
        }
        return ans;
    }
}