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.

思路：

每個點的邊為4，相鄰的兩個點共享一條邊，所以每次碰到連接處，邊的個數減去2。我們從左上掃到右下，所以可以只考慮右邊和下邊的邊。

題解：

int islandPerimeter(const std::vector<std::vector<int>>& grid) {    const size_t M = grid.size();    const size_t N = grid[0].size();    int perimeter(0);    for(size_t i = 0; i < M; ++i) {        for(size_t j = 0; j < N; ++j) {            if (grid[i][j]) {                perimeter += 4;                if (i != M - 1 && grid[i + 1][j]) perimeter -= 2;                if (j != N - 1 && grid[i][j + 1]) perimeter -= 2;            }        }    }    return perimeter;}