463. Island Perimeter

463. Island Perimeter

You are given a map in form of a two-dimensional integer grid where 1 represents land and 0 represents water. Grid cells are connected horizontally/vertically (not diagonally). The grid is completely surrounded by water, and there is exactly one island (i.e., one or more connected land cells). The island doesn't have "lakes" (water inside that isn't connected to the water around the island). One cell is a square with side length 1. The grid is rectangular, width and height don't exceed 100. Determine the perimeter of the island.

Example:

[[0,1,0,0],
 [1,1,1,0],
 [0,1,0,0],
 [1,1,0,0]]

Answer: 16
Explanation: The perimeter is the 16 yellow stripes in the image below:

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数1周边是0的个数总和

class Solution {
    public int islandPerimeter(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;
        
        int perimeter = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 1) {
                    perimeter += countEdge(grid, i, j);
                }
            }
        }
        
        return perimeter;
    }
    
    private int countEdge(int[][] grid, int row, int col) {
        return helper(grid, row + 1, col) + helper(grid, row, col + 1) 
            + helper(grid, row, col - 1) + helper(grid, row - 1, col);
    }
    
    private int helper(int[][] grid, int row, int col) {
        if (row >= 0 && row < grid.length && col >= 0 && col < grid[0].length && grid[row][col] == 0) {
            return 1;
        }
        if (row == -1 || row == grid.length || col == -1 || col == grid[0].length) {
            return 1;
        }
        return 0;
    }
}

另一种方法是计1的个数为j，对每一个1都找出它周边是1的个数总和k，最终4 * j - 2 * k