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:

本题求不规则正方形的周长，开始的时候没有想出来，看了答案才知道；想法是遍历到某一位置为1时候，islands+1，同时检测其下方和右面是否为1，如果为1，则neighbor+1；代码如下：

public class Solution {
    public int islandPerimeter(int[][] grid) {
        int islands = 0;
        int neighbors = 0;
        for(int i=0;i<grid.length;i++){
            for(int j=0;j<grid[i].length;j++){
                if(grid[i][j]==1){
                    islands++;
                    if(i<grid.length-1&&grid[i+1][j]==1) neighbors++;
                    if(j<grid[i].length-1&&grid[i][j+1]==1) neighbors++;
                }
            }
        }
        return islands*4-2*neighbors;
    }
}