Leetcode463. Island Perimeter

Leetcode463. Island Perimeter

题目：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:

题目分析：
检测黄色竖条有多少个，大概的意思就是用一个二次循环来遍历，检测到黄色板块的时候，再判断该黄色板块上下左右有多少个黄色板块（或检测蓝色板块），然后4-黄色板块个数就是该黄色板块的黄色竖条的个数。
题目还是相对简单的，但是由于时间复杂度是O(n^2)，所以也尝试做了一些优化，但是还是没有办法，也看了很多博客，也大多数都是用的二次循环。

下面是我的代码：

class Solution {
public:
int islandPerimeter(vector<vector<int>>& grid) {
int sum = 0;
for (int i = 0; i < grid.size(); i++) {
for (int j = 0; j < grid[i].size(); j++) {
if (grid[i][j] == 1) {
int tmp = 0;
if (i - 1 >= 0) tmp += grid[i - 1][j];
if (i + 1 < grid.size()) tmp += grid[i + 1][j];
if (j - 1 >= 0) tmp += grid[i][j - 1];
if (j + 1 < grid[i].size()) tmp += grid[i][j + 1];
sum += 4 - tmp;
}
}
}
return sum;
}
};