Leetcode: Island Perimeter

Island Perimeter

Similar Problems:

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.



Answer: 16

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## Blog link: https://code.dennyzhang.com/island-perimeter
class Solution(object):
    def islandPerimeter(self, grid):
        :type grid: List[List[int]]
        :rtype: int
        row_count = len(grid)
        if row_count==0: return 0
        col_count = len(grid[0])
        cell_count, neighbor_count = 0, 0
        for i in xrange(row_count):
            for j in xrange(col_count):
                if grid[i][j] == 1:
                    cell_count += 1
                    # only count the right and down
                    if j != col_count-1 and grid[i][j+1] == 1: neighbor_count += 1
                    if i != row_count-1 and grid[i+1][j] == 1: neighbor_count += 1
        return cell_count*4 - 2*neighbor_count

    ## Basic Idea:  Get how many 1 cells. Let's say it's m
    ##              Find how many 1-1 pair which is adjacent, let's say it's n
    ##              The result is 4*m-2*n
    ## Complexity: Time O(m*n), Space O(1)
    def islandPerimeter_v1(self, grid):
        :type grid: List[List[int]]
        :rtype: int
        self.row_count = len(grid)
        if self.row_count==0: return 0
        self.col_count = len(grid[0])

        cell_count = 0
        for i in xrange(self.row_count):
            for j in xrange(self.col_count):
                if grid[i][j] == 1:
                    cell_count += 1

        self.adjacent_count = 0
        for i in xrange(self.row_count):
            for j in xrange(self.col_count):
                if grid[i][j] == 1:
                    self.DFSMark(grid, i, j)
                    return 4*cell_count - self.adjacent_count

    def DFSMark(self, grid, i, j):
        if self.isValidIndex(i, j) is False: return
        if grid[i][j] != 1: return

        grid[i][j] = 2
        self.adjacent_count += self.GetAdjacentCellCount(grid, i, j)
        self.DFSMark(grid, i-1, j)
        self.DFSMark(grid, i+1, j)
        self.DFSMark(grid, i, j-1)
        self.DFSMark(grid, i, j+1)

    def isValidIndex(self, i, j):
        return not(i<0 or i>=self.row_count or \
                    j<0 or j>=self.col_count)

    def GetAdjacentCellCount(self, grid, i, j):
        count = 0
        if self.isValidIndex(i-1, j) and grid[i-1][j] != 0: count += 1
        if self.isValidIndex(i+1, j) and grid[i+1][j] != 0: count += 1
        if self.isValidIndex(i, j-1) and grid[i][j-1] != 0: count += 1
        if self.isValidIndex(i, j+1) and grid[i][j+1] != 0: count += 1
        # print i, j, count
        return count

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