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.

樣例

[[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:

解題思路：

遍歷矩陣，若遇到1的情況，則需要判斷加四條邊的哪幾條邊，或者不加，這與它的四鄰邊有關係。具體表現為：如果它左邊為0，或者它左邊已經是邊界的情況，表明存在這條邊，周長可以加上1，右上下邊類比即可。

public class Solution {
    /**
     * @param grid: a 2D array
     * @return: the perimeter of the island
     */
    public int islandPerimeter(int[][] grid) {
        // Write your code here
        int perimeter = 0;
        
        for(int i=0 ; i<grid.length ; i++){
            for(int j=0 ; j<grid[0].length ; j++){
                if(grid[i][j] == 1){
                    if((j-1)<0 || grid[i][j-1]==0) //左邊
                        perimeter++;
                    if((j+1)>=grid[0].length || grid[i][j+1]==0) //右邊
                        perimeter++;
                    if((i-1)<0 || grid[i-1][j]==0) //上邊
                        perimeter++;
                    if((i+1)>=grid.length || grid[i+1][j]==0) //下邊
                        perimeter++;
                }
            }
        }
        
        return perimeter;
    }
}