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:

参考的大牛的思路：

1. find how many 1 in the map. If without the consideration of surrounding cells, the total perimeter should be the total amount of 1 times 4.
2. find how many cell walls that connect with both lands. We need to deduct twice of those lines from total perimeter

int islandPerimeter(vector<vector<int>>& grid)
     {
         int count = 0, repeat = 0;
         for (int i = 0; i < grid.size(); i++)
         {
             for (int j = 0; j < grid[i].size(); j++)
             {
                 if (grid[i][j] == 1)
                 {
                     count++;
                     if (i != 0 && grid[i - 1][j] == 1) repeat++;
                     if (j != 0 && grid[i][j - 1] == 1) repeat++;
                 }
             }
         }
         return 4 * count - repeat * 2;
     }

参考:

https://discuss.leetcode.com/topic/68845/c-solution-with-explanation

[leetcode-463-Island Perimeter]