# Leetcode: Number of Islands

Number of Islands Similar Problems:

Given a 2d grid map of ‘1’s (land) and ‘0’s (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

```Example 1:

11110
11010
11000
00000
```
```Example 2:

11000
11000
00100
00011
```

Github: code.dennyzhang.com

Credits To: leetcode.com

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

```## Blog link: https://code.dennyzhang.com/number-of-islands
class Solution(object):
## Basic Ideas: Avoid duplicate counting.
##   Mark all adjacent 1 to X. Thus we can avoid counting one the same island multiple times.
##
## Complexity: Time O(m*n), Space O(1)
def numIslands(self, grid):
"""
:type grid: List[List[str]]
:rtype: int
"""
self.row_count = len(grid)
if self.row_count == 0: return 0
self.col_count = len(grid)

res = 0
for i in xrange(self.row_count):
for j in xrange(self.col_count):
if grid[i][j] == '1':
res += 1
self.DFSMark(grid, i, j)
return res

def DFSMark(self, grid, i, j):
if i < 0 or i >= self.row_count \
or j < 0 or j >= self.col_count:
return

# stop digging, if not '1'
if grid[i][j] != '1': return

grid[i][j] = 'X'
# mark four positions in a recursive way
self.DFSMark(grid, i-1, j)
self.DFSMark(grid, i+1, j)
self.DFSMark(grid, i, j-1)
self.DFSMark(grid, i, j+1)
```

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