695. Max Area of Island

lc695.md

@@ -0,0 +1,158 @@
+# step1
+思考ログ
+- 前回のやつとかなり似ている問題
+- dfsとbfsでどの場面でどう使い分けるのかにピンときていない.
+- とりあえず再帰で解こうかな→再帰でやっているけど沼って解けない
+- 再帰を使わずに解いた方が簡単な気がしたので方針転換
+- 前回の問題と違って, 0と1がstrではなくintであることに気付かず混乱したが, なんとか解けた.
+
+```python
+class Solution:
+    def maxAreaOfIsland(self, grid: List[List[int]]) -> int:
+        WATER = 0
+        ISLAND = 1
+        visited_island = set()
+
+        num_rows = len(grid)
+        num_columns = len(grid[0])
+
+        def inside_island(i, j):
+            return 0 <= i < num_rows and 0 <= j < num_columns
+
+        def traverse_island(area, row, column):
+            island = deque([(row, column)])
+
+            while island:
+                r, c = island.popleft()
+                for (i, j) in ((0, 0), (1, 0), (-1, 0), (0, 1), (0, -1)):
+                    if not inside_island(r + i, c + j):
+                        continue
+                    elif grid[r + i][c + j] == WATER:
+                        continue
+                    elif (r + i, c + j) in visited_island:
+                        continue
+
+                    visited_island.add((r + i, c + j))
+                    island.append((r + i, c + j))
+                    area += 1
+
+            return area
+
+
+        max_area = 0
+        for i in range(num_rows):
+            for j in range(num_columns):
+                area = 0
+                if grid[i][j] == WATER:
+                    continue
+                elif (i, j) in visited_island:
+                    continue
+                area = traverse_island(area, i, j)
+                max_area = max(area, max_area)
+
+        return max_area
+```
+
+# step2
+- 他の方の解法を参考にする
+- https://github.com/potrue/leetcode/pull/17/files?short_path=3c1ad9a#diff-3c1ad9a7fa436a1a165df66488e8abca67c442a6c1b268a312d6be0ea6c169c8
+- ここの[discussion](https://github.com/potrue/leetcode/pull/17/files#r2103404510)は前回の問題でi, jをごちゃごちゃ書いていたのもあり意識するべきだと感じた
+- https://github.com/plushn/SWE-Arai60/pull/18#discussion_r2099839166
+
+- https://github.com/plushn/SWE-Arai60/pull/18/files?short_path=2e957ae#diff-2e957aed1bb9dc5099abe16f37afb1a7980cfb2170d04ad76a48b4566e75eb35
+- step1で再帰で実装するのを諦めたが際に, この方のstep1をみて実装のミスに気づいた.- [再帰のコールスタック制限を変更するsetrecursionlimit](https://docs.python.org/ja/3/library/sys.html#sys.setrecursionlimit)を見たが, 結局これを設定するのにはどうすれば良いのかわからない. とりあえず知識として持っておこうと思う
+
+- https://github.com/tokuhirat/LeetCode/pull/18/files?short_path=810fef1#diff-810fef11514f386f3a16aee6147492af7b7b23c593173810d148acac46695309
+- https://github.com/shintaro1993/arai60/pull/22/files?short_path=83c7941#diff-83c7941d06b952f77f8ed25bdf52322f3588314fb73e5da782120289b2c4314f
+
+# step3
+- step1を少し修正
+```python
+class Solution:
+    def maxAreaOfIsland(self, grid: List[List[int]]) -> int:
+        WATER = 0
+        ISLAND = 1
+        visited_island = set()
+
+        num_rows = len(grid)
+        num_columns = len(grid[0])
+
+        def inside_island(row, column):
+            return 0 <= row < num_rows and 0 <= column < num_columns
+
+        def traverse_and_count_island(row, column):
+            island = deque([(row, column)])
+            area = 1
+            visited_island.add((row, column))
+
+            while island:
+                r, c = island.popleft()
+                for (i, j) in ((1, 0), (-1, 0), (0, 1), (0, -1)):
+                    if not inside_island(r + i, c + j):
+                        continue
+                    elif grid[r + i][c + j] == WATER:
+                        continue
+                    elif (r + i, c + j) in visited_island:
+                        continue
+                    area += 1
+                    island.append((r + i, c + j))
+                    visited_island.add((r + i, c + j))
+
+            return area
+
+        max_area = 0
+        for i in range(num_rows):
+            for j in range(num_columns):
+                if grid[i][j] == WATER:
+                    continue
+                elif (i, j) in visited_island:
+                    continue
+                area = traverse_and_count_island(i, j)
+                max_area = max(area, max_area)
+
+        return max_area
+```
+- 再帰でも解いてみた
+```python
+class Solution:
+    def maxAreaOfIsland(self, grid: List[List[int]]) -> int:
+        WATER = 0
+        ISLAND = 1
+        visited_island = set()
+
+        num_rows = len(grid)
+        num_columns = len(grid[0])
+
+        def inside_island(row, column):
+            return 0 <= row < num_rows and 0 <= column < num_columns
+
+        def traverse_and_count_island(row, column):
+            if not inside_island(row, column):
+                return 0
+            elif grid[row][column] == WATER:
+                return 0
+            elif (row, column) in visited_island:
+                return 0
+
+            visited_island.add((row, column))
+            area = 1
+            area += traverse_and_count_island(row + 1, column)
+            area += traverse_and_count_island(row - 1, column)
+            area += traverse_and_count_island(row, column + 1)
+            area += traverse_and_count_island(row, column - 1)
+
+            return area
+
+
+        max_area = 0
+        for i in range(num_rows):
+            for j in range(num_columns):
+                if grid[i][j] == WATER:
+                    continue
+                elif (i, j) in visited_island:
+                    continue
+                area = traverse_and_count_island(i, j)
+                max_area = max(area, max_area)
+
+        return max_area
+```