# 463. Island Perimeter

### 題目敘述

You are given `row x col` `grid` representing a map where `grid[i][j] = 1` represents land and `grid[i][j] = 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", meaning the water inside 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 1:

``````Input: grid = [[0,1,0,0],[1,1,1,0],[0,1,0,0],[1,1,0,0]]
Output: 16
Explanation: The perimeter is the 16 yellow stripes in the image above.
``````

Example 2:

``````Input: grid = [[1]]
Output: 4
``````

Example 3:

``````Input: grid = [[1,0]]
Output: 4
``````

Constraints:

• `row == grid.length`
• `col == grid[i].length`
• `1 <= row, col <= 100`
• `grid[i][j]` is `0` or `1`.
• There is exactly one island in `grid`.

### 解法解析

Time complexity：`O(mn)` Space complexity：`O(1)`

#### 程式範例

##### Python
``````class Solution:
def islandPerimeter(self, grid: List[List[int]]) -> int:
perimeter = 0
for i in range(len(grid)):
for j, val in enumerate(grid[i]):
if val:
perimeter += 4

if i > 0 and grid[i-1][j]:
perimeter -= 2
if j > 0 and grid[i][j - 1]:
perimeter -= 2
return perimeter
``````
##### JavaScript
``````/**
* @param {number[][]} grid
* @return {number}
*/
var islandPerimeter = function (grid) {
let perimeter = 0;

for (let i = 0; i < grid.length; i++) {
for (let j = 0; j < grid[i].length; j++) {
if (grid[i][j]) {
perimeter += 4;

if (i > 0 && grid[i - 1][j]) {
perimeter -= 2;
}
if (j > 0 && grid[i][j - 1]) {
perimeter -= 2;
}
}
}
}
return perimeter;
};
``````
##### Go
``````func islandPerimeter(grid [][]int) int {
var perimeter int
for i := 0; i < len(grid); i++ {
for j := 0; j < len(grid[0]); j++ {
if grid[i][j] == 1 {
perimeter += 4
if i > 0 && grid[i-1][j] == 1 {
perimeter -= 2
}
if j > 0 && grid[i][j-1] == 1 {
perimeter -= 2
}
}
}
}
return perimeter
}
``````
##### Swift
``````class Solution {
func islandPerimeter(_ grid: [[Int]]) -> Int {
var perimeter = 0
for ridx in 0..<grid.count {
for cidx in 0..<grid[0].count {
if grid[ridx][cidx] == 0 { continue }

let prev = cidx > 0 && grid[ridx][cidx - 1] == 1 ? 2 : 0
let top = ridx > 0 && grid[ridx - 1][cidx ] == 1 ? 2 : 0

perimeter += 4 - (prev + top)
}
}
return perimeter
}
}
``````
##### Kotlin
``````class Solution {
fun islandPerimeter(grid: Array<IntArray>): Int {
var perimeter = 0
for (i in grid.indices) {
for (j in grid[i].indices) {
if (grid[i][j] == 1) {
perimeter += 4
if (i > 0 && grid[i - 1][j] == 1) {
perimeter -= 2
}
if (j > 0 && grid[i][j - 1] == 1) {
perimeter -= 2
}
}
}
}
return perimeter
}
}
``````