# Leetcode 463. Island Perimeter 題解

## 逐一計算

class Solution:
    def islandPerimeter(self, grid: List[List[int]]) -> int:
        xc,yc = len(grid[0]), len(grid)
        output = 0
        for y in range(yc):
            for x in range(xc):
                if grid[y][x] == 1:
                    borders = 4
                    if x >= 1:
                        if grid[y][x-1] == 1: # 左
                            borders -= 1
                    if x + 1 < xc:
                        if grid[y][x+1] == 1: # 右
                            borders -= 1
                    if y >= 1:
                        if grid[y-1][x] == 1: # 上
                            borders -= 1
                    if y + 1 < yc:
                        if grid[y+1][x] == 1: # 下
                            borders -= 1
                    output += borders
        return output

## 計算右下相鄰島


class Solution:
    def islandPerimeter(self, grid: List[List[int]]) -> int:
        xc, yc = len(grid[0]), len(grid)
        islands, neighbours = 0, 0
        for y in range(yc):
            for x in range(xc):
                if grid[y][x] == 1:
                    islands += 1
                    if x + 1 < xc:
                        if grid[y][x + 1] == 1: # 右
                            neighbours += 1
                    if y + 1 < yc:
                        if grid[y + 1][x] == 1: # 下
                            neighbours += 1
        return islands * 4 - neighbours * 2

### 2024.04.18 每日

class Solution:
    def islandPerimeter(self, grid: List[List[int]]) -> int:
        m,n = len(grid), len(grid[0])
        dem = [[0,-1], [0,1], [-1,0], [1,0]]
        perimeters = 0
        for y in range(m):
            for x in range(n):
                perimeters += grid[y][x] * 4
                if grid[y][x] == 1:
                    for dx,dy in dem:
                        dx = x + dx
                        dy = y + dy
                        if 0 <= dx < n and 0 <= dy < m:
                            perimeters -= grid[dy][dx]
        return perimeters