leetcode 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:

统计相邻的区域个数，减去相交边即可，统计四个方向的方法好像慢了点，可以只查看右和下两个方向的，示例见下下方代码

public class Solution {
    public int islandPerimeter(int[][] grid) {
        int counts = 0;
        for(int i=0; i<grid.length; i++){
            for(int j=0; j<grid[0].length; j++){
                if(grid[i][j]==1)
                    counts += 4 - adjacentNums(grid, i, j);
            }
        }
        return counts;
    }
    
    private int adjacentNums(int[][] grid, int i, int j){
        int count=0;
        if(i-1>=0) if(grid[i-1][j]==1) count++;
        if(i+1<grid.length) if(grid[i+1][j]==1) count++;
        if(j-1>=0) if(grid[i][j-1]==1) count++;
        if(j+1<grid[0].length) if(grid[i][j+1]==1) count++;
        return count;
    } 
}

示例

public class Solution {
    public int islandPerimeter(int[][] grid) {
        int islands = 0, neighbours = 0;

        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[i].length; j++) {
                if (grid[i][j] == 1) {
                    islands++; // count islands
                    if (i < grid.length - 1 && grid[i + 1][j] == 1) neighbours++; // count down neighbours
                    if (j < grid[i].length - 1 && grid[i][j + 1] == 1) neighbours++; // count right neighbours
                }
            }
        }

        return islands * 4 - neighbours * 2;
    }
}