[LeetCode] Island Perimeter

Posted 2020-09-27 immjc

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:

题目要求由1围成图形的周长，图形的周长 = 全部1的个数 * 4 - 相邻1的个数 * 2。只要求出全部1的个数和相邻1的个数即可。遍历grid，全部1的个数很好计算，相邻1的个数只需要在一个1存在的情况下判断该1的上边和左边是否存在其他的1。

class Solution {
public:
    int islandPerimeter(vector<vector<int>>& grid) {
        int cnt = 0, rep = 0;
        for (int i = 0; i != grid.size(); i++)
            for (int j = 0; j != grid[0].size(); j++) 
                if (grid[i][j] == 1) {
                    cnt++;
                    if (i != 0 && grid[i - 1][j] == 1)
                        rep++;
                    if (j != 0 && grid[i][j - 1] == 1)
                        rep++;
                }
        return cnt * 4 - rep * 2;
    }
};
// 139 ms