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:

解题思路：
1.计算island个数
2.计算island邻居数（右邻居＋下邻居）
3.有1个邻居消失2条边

Language－Java, Time－O(n), Space－O(1), RunTime－145ms

public int islandPerimeter(int[][] grid) {        int count1 = 0; //island个数        int count2 = 0; //邻居个数        for(int i = 0; i < grid.length; i ++)        {            for(int j = 0; j < grid[i].length; j ++)            {                if(grid[i][j] == 1)                {                    count1++;                    if(i < grid.length - 1 && grid[i+1][j] == 1) count2++; //有下邻居                    if(j < grid[i].length - 1 && grid[i][j+1] == 1) count2++; //有右邻居                }            }        }        return count1 * 4 - count2 *2; //有1个邻居消失2条边    }