From c544dcadf55b78524a25f44ea0537aa9c6192ab6 Mon Sep 17 00:00:00 2001
From: Hiro Funatsuka <hiro.funatsuka@gmail.com>
Date: Mon, 25 May 2026 23:29:52 +0900
Subject: [PATCH] Solve number of islands

---
 200_number_of_islands/problem.md |  1 +
 200_number_of_islands/step1.md   | 36 ++++++++++++++++++++++++++++++++
 200_number_of_islands/step2.md   | 34 ++++++++++++++++++++++++++++++
 200_number_of_islands/step3.md   | 33 +++++++++++++++++++++++++++++
 4 files changed, 104 insertions(+)
 create mode 100644 200_number_of_islands/problem.md
 create mode 100644 200_number_of_islands/step1.md
 create mode 100644 200_number_of_islands/step2.md
 create mode 100644 200_number_of_islands/step3.md

diff --git a/200_number_of_islands/problem.md b/200_number_of_islands/problem.md
new file mode 100644
index 0000000..1e2627f
--- /dev/null
+++ b/200_number_of_islands/problem.md
@@ -0,0 +1 @@
+## 問題: [200. Number of Islands](https://leetcode.com/problems/number-of-islands/description/)
diff --git a/200_number_of_islands/step1.md b/200_number_of_islands/step1.md
new file mode 100644
index 0000000..8eb41e3
--- /dev/null
+++ b/200_number_of_islands/step1.md
@@ -0,0 +1,36 @@
+# Step 1
+
+- 各セルで条件を満たす場合、dfsを行う
+  - dfs
+    - セルが1、まだ訪れていない、かつ範囲内だったら、counterを1増やして上下左右に行き、次のセルでも同じことを繰り返す
+
+```python
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        rows = len(grid)
+        columns = len(grid[0])
+        visited = set()
+        num_islands = 0
+
+        def dfs(r, c):
+            if (r, c) not in visited and r >= 0 and r < rows and c >= 0 and c < columns and grid[r][c] == "1":
+                visited.add((r, c))
+                dfs(r + 1, c)
+                dfs(r - 1, c)
+                dfs(r, c + 1)
+                dfs(r, c - 1)
+
+        for r in range(rows):
+            for c in range(columns):
+                if grid[r][c] == "1" and (r, c) not in visited:
+                    num_islands += 1
+                    dfs(r, c)
+
+        return num_islands
+```
+
+時間計算量: $O(mn)$
+
+空間計算量: $O(mn)$
+
+アルゴリズム最大実行時間(概算): 時間計算量 / Pythonの1秒あたりの計算ステップ数 -> $(300 \times 300) \div 10,000,000 = 0.001 (s)$
diff --git a/200_number_of_islands/step2.md b/200_number_of_islands/step2.md
new file mode 100644
index 0000000..7a2008f
--- /dev/null
+++ b/200_number_of_islands/step2.md
@@ -0,0 +1,34 @@
+# Step 2
+
+```python
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        rows = len(grid)
+        columns = len(grid[0])
+        directions = [[1, 0], [-1, 0], [0, 1], [0, -1]]
+        visited = set()
+        num_islands = 0
+
+        def dfs(r, c):
+            if not (0 <= r < rows and 0 <= c < columns):
+                return
+            if grid[r][c] != "1" or (r, c) in visited:
+                return
+            visited.add((r, c))
+            for dr, dc in directions:
+                dfs(r + dr, c + dc)
+
+        for r in range(rows):
+            for c in range(columns):
+                if grid[r][c] == "1" and (r, c) not in visited:
+                    num_islands += 1
+                    dfs(r, c)
+
+        return num_islands
+```
+
+時間計算量: $O(mn)$
+
+空間計算量: $O(mn)$
+
+アルゴリズム最大実行時間(概算): 時間計算量 / Pythonの1秒あたりの計算ステップ数 -> $(300 \times 300) \div 10,000,000 = 0.001 (s)$
diff --git a/200_number_of_islands/step3.md b/200_number_of_islands/step3.md
new file mode 100644
index 0000000..1153ea5
--- /dev/null
+++ b/200_number_of_islands/step3.md
@@ -0,0 +1,33 @@
+# Step 3
+
+```python
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        rows = len(grid)
+        columns = len(grid[0])
+        directions = [[1, 0], [-1, 0], [0, 1], [0, -1]]
+        visited = set()
+        num_islands = 0
+
+        def dfs(r, c):
+            if not (0 <= r < rows and 0 <= c < columns):
+                return
+            if grid[r][c] != "1" or (r, c) in visited:
+                return
+            visited.add((r, c))
+            for dr, dc in directions:
+                dfs(r + dr, c + dc)
+
+        for r in range(rows):
+            for c in range(columns):
+                if grid[r][c] == "1" and (r, c) not in visited:
+                    num_islands += 1
+                    dfs(r, c)
+
+        return num_islands
+```
+1回目: 5分　9秒
+
+2回目: 3分　25秒
+
+3回目: 2分　57秒