Flood Fill

0733.Flood-Fill/memo.md

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+# 733. Flood Fill
+
+## step1
+in-placeだが良いだろう
+
+DFS
+
+## step2
+
+他の人のコード
+
+https://github.com/huyfififi/coding-challenges/pull/9
+
+- キュー／スタックに入れる前に色を塗る／範囲を先に見ることで重複探索を防ぐ（DFS で「呼び出し後に塗る」と無限再帰になりうる）
+
+https://github.com/ryosuketc/leetcode_grind75/pull/9
+
+https://github.com/Kitaken0107/GrindEasy/pull/12
+
+
+同じBFS/DFSでもキューに入れる前に条件で省くか、とりあえず入れておくかの二通りがある。
+
+個人的には無駄な操作を省く前者の方が好み
+
+読みやすさ的には後者を好む人もいそう
+
+
+
+
+
+
+> 3回passするようになると、コードがちょうどいい抽象度で頭に収まるような実感がある。
+
+サボっているので真面目にやるべきなのか...
+
+seenが不要なのでその点を改良する
+
+## step3
+
+個人的に馴染みのない、BFS+とりあえず入れておく解法を書く

0733.Flood-Fill/step1.py

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+class Solution:
+    def floodFill(
+        self, image: List[List[int]], sr: int, sc: int, color: int
+    ) -> List[List[int]]:
+        base_color = image[sr][sc]
+        seen = [[False] * len(image[0]) for _ in range(len(image))]
+        stack = [(sr, sc)]
+        seen[sr][sc] = True
+
+        while stack:
+            row, col = stack.pop()
+            image[row][col] = color
+            for row_next, col_next in (
+                (row, col + 1),
+                (row, col - 1),
+                (row + 1, col),
+                (row - 1, col),
+            ):
+                if not (0 <= row_next < len(image) and 0 <= col_next < len(image[0])):
+                    continue
+                if (
+                    not seen[row_next][col_next]
+                    and image[row_next][col_next] == base_color
+                ):
+                    stack.append((row_next, col_next))
+                    seen[row_next][col_next] = True
+
+        return image

0733.Flood-Fill/step2.py

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+import collections
+
+
+class Solution:
+    def floodFill(
+        self, image: List[List[int]], sr: int, sc: int, color: int
+    ) -> List[List[int]]:
+        """BFS: 四方向はとりあえず全部キューに入れ、取り出したあと境界と元色を見る。"""
+        base_color = image[sr][sc]
+        if base_color == color:
+            return image
+
+        num_rows = len(image)
+        num_cols = len(image[0])
+        frontier = collections.deque([(sr, sc)])
+
+        while frontier:
+            row, col = frontier.popleft()
+            if not (0 <= row < num_rows and 0 <= col < num_cols):
+                continue
+            if image[row][col] != base_color:
+                continue
+            image[row][col] = color
+            frontier.append((row - 1, col))
+            frontier.append((row + 1, col))
+            frontier.append((row, col - 1))
+            frontier.append((row, col + 1))
+
+        return image
+
+
+class Solution:
+    def floodFill(
+        self, image: List[List[int]], sr: int, sc: int, color: int
+    ) -> List[List[int]]:
+        """BFS（deque）、キューに多重投入されても、取り出し時に base_color でないマスは skip。"""
+        base_color = image[sr][sc]
+        if base_color == color:
+            return image
+
+        num_rows = len(image)
+        num_cols = len(image[0])
+        directions = (
+            (-1, 0),
+            (1, 0),
+            (0, -1),
+            (0, 1),
+        )
+        frontier = collections.deque([(sr, sc)])
+
+        while frontier:
+            row, col = frontier.popleft()
+            if image[row][col] != base_color:
+                continue
+            image[row][col] = color
+            for dr, dc in directions:
+                row_next, col_next = row + dr, col + dc
+                if (
+                    0 <= row_next < num_rows
+                    and 0 <= col_next < num_cols
+                    and image[row_next][col_next] == base_color
+                ):
+                    frontier.append((row_next, col_next))
+
+        return image
+
+
+class Solution:
+    def floodFill(
+        self, image: List[List[int]], sr: int, sc: int, color: int
+    ) -> List[List[int]]:
+        """DFS 再帰"""
+        base_color = image[sr][sc]
+        if base_color == color:
+            return image
+
+        num_rows = len(image)
+        num_cols = len(image[0])
+
+        def paint_region(row: int, col: int) -> None:
+            if not (0 <= row < num_rows and 0 <= col < num_cols):
+                return
+            if image[row][col] != base_color:
+                return
+            image[row][col] = color
+            for dr, dc in (
+                (-1, 0),
+                (1, 0),
+                (0, -1),
+                (0, 1),
+            ):
+                paint_region(row + dr, col + dc)
+
+        paint_region(sr, sc)
+        return image

0733.Flood-Fill/step3.py

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+import collections
+
+
+class Solution:
+    def floodFill(
+        self, image: List[List[int]], sr: int, sc: int, color: int
+    ) -> List[List[int]]:
+        color_to_update = image[sr][sc]
+        if color_to_update == color:
+            return image
+
+        frontier = collections.deque([(sr, sc)])
+
+        while frontier:
+            row, col = frontier.popleft()
+            if not (0 <= row < len(image) and 0 <= col < len(image[0])):
+                continue
+            if image[row][col] != color_to_update:
+                continue
+            image[row][col] = color
+
+            for next_row, next_col in (
+                (row, col + 1),
+                (row, col - 1),
+                (row + 1, col),
+                (row - 1, col),
+            ):
+                frontier.append((next_row, next_col))
+
+        return image