LeetCode 463. Island Perimeter (算法, 矩阵遍历)

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:

输入一个矩阵，1代表陆地，0代表水，计算岛屿的周长。

思路：若不考虑每块陆地的相邻条件，则一块陆地的周长为4。两块陆地相邻，则周长减少2。

逐个元素遍历矩阵，查找每个元素左侧、上方是否有相邻的1（两元素相乘为1）。

class Solution {
public:
intislandPerimeter(vector<vector<int>>& grid) {
int count = 0,neighbor = 0;
for (int i=0;i<grid.size();i++)
{
for (int j=0; j<grid[0].size();j++)
{
if(grid[i][j]==1)count++;
if(i>0 && grid[i][j]*grid[i-1][j]==1)neighbor++;
if(j>0 && grid[i][j]*grid[i][j-1]==1)neighbor++;
}
}
return count*4-neighbor*2;
}
};