463. Island Perimeter

ou 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:

题目含义：给定一个二维地图，1表示陆地，0表示水域。单元格水平或者竖直相连（不含对角线）。地图完全被水域环绕，只包含一个岛屿（也就是说，一个或者多个相连的陆地单元格）。岛屿没有湖泊（岛屿内部环绕的水域）。单元格是边长为1的正方形。地图是矩形，长宽不超过100。计算岛屿的周长。

 

    public int islandPerimeter(int[][] grid) {
//        对于每个岛屿格子先默认加上四条边，然后检查其左面和上面是否有岛屿格子，有的话分别减去两条边，这样也能得到正确的结果，
        int m = grid.length;
        int n = grid[0].length;
        if (m == 0 || n == 0) return 0;
        int sum = 0;
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 0) continue;
                sum += 4;
                if (i > 0 && grid[i - 1][j] == 1) sum -= 2;
                if (j > 0 && grid[i][j - 1] == 1) sum -= 2;
            }

        return sum;    
    }