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:

image

思路:先數有幾個land(棕色),不考慮重復邊的話,總邊數=land4.
然而有重復邊,即相鄰兩個land的公共邊被重復計算了兩次,記總的重復邊為repeat,則最終結果為land4-repeat*2

class Solution {
public:
    int islandPerimeter(vector<vector<int>>& grid) {
        int count = 0; // land計數
        int repeat = 0; // 重復邊計數
        int row = grid.size();
        int col = grid[0].size();
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (grid[i][j] == 1) {
                    count++; // land計數+1
                    // 若找到一個land,檢查其右邊和下方的區域是否也為land,若是,則重復邊+1
                    // 注意i和j不要越界
                    if (j != row-1 && grid[i][j+1] == 1) repeat++;
                    if (i != col-1 && grid[i+1][j] == 1) repeat++;
                }
            }
        }
        return count*4 - repeat*2;
    }
};