diff --git a/200-number-of-islands/memo.md b/200-number-of-islands/memo.md
new file mode 100644
index 0000000..0ed407d
--- /dev/null
+++ b/200-number-of-islands/memo.md
@@ -0,0 +1,12 @@
+### step1
+
+実装方法がわからなかったのでChatGPTを利用。
+for文を回して'1'のマスに当たるたびに、countを増やしてdfsで隣接するマスをすべて'0'にすることを繰り返すだけだった。
+
+### step2
+
+変更点なし。
+
+### step3
+
+3回通すまで書き直し。
diff --git a/200-number-of-islands/step1.cpp b/200-number-of-islands/step1.cpp
new file mode 100644
index 0000000..438a264
--- /dev/null
+++ b/200-number-of-islands/step1.cpp
@@ -0,0 +1,32 @@
+class Solution {
+public:
+    int m, n;
+
+    void dfs(vector<vector<char>>& grid, int x, int y) {
+        if (x < 0 || x >= m || y < 0 || y >= n || grid[x][y] == '0') {
+            return;
+        }
+        grid[x][y] = '0';
+
+        dfs(grid, x + 1, y);
+        dfs(grid, x - 1, y);
+        dfs(grid, x, y + 1);
+        dfs(grid, x, y - 1);
+    }
+    
+    int numIslands(vector<vector<char>>& grid) {
+        m = grid.size();
+        n = grid[0].size();
+        int count = 0;
+
+        for(int i = 0; i < m; i++) {
+            for(int j = 0; j < n; j++) {
+                if (grid[i][j] == '1') {
+                    count++;
+                    dfs(grid, i, j);
+                }
+            }
+        }
+        return count;
+    }
+};
diff --git a/200-number-of-islands/step2.cpp b/200-number-of-islands/step2.cpp
new file mode 100644
index 0000000..438a264
--- /dev/null
+++ b/200-number-of-islands/step2.cpp
@@ -0,0 +1,32 @@
+class Solution {
+public:
+    int m, n;
+
+    void dfs(vector<vector<char>>& grid, int x, int y) {
+        if (x < 0 || x >= m || y < 0 || y >= n || grid[x][y] == '0') {
+            return;
+        }
+        grid[x][y] = '0';
+
+        dfs(grid, x + 1, y);
+        dfs(grid, x - 1, y);
+        dfs(grid, x, y + 1);
+        dfs(grid, x, y - 1);
+    }
+    
+    int numIslands(vector<vector<char>>& grid) {
+        m = grid.size();
+        n = grid[0].size();
+        int count = 0;
+
+        for(int i = 0; i < m; i++) {
+            for(int j = 0; j < n; j++) {
+                if (grid[i][j] == '1') {
+                    count++;
+                    dfs(grid, i, j);
+                }
+            }
+        }
+        return count;
+    }
+};
diff --git a/200-number-of-islands/step3.cpp b/200-number-of-islands/step3.cpp
new file mode 100644
index 0000000..438a264
--- /dev/null
+++ b/200-number-of-islands/step3.cpp
@@ -0,0 +1,32 @@
+class Solution {
+public:
+    int m, n;
+
+    void dfs(vector<vector<char>>& grid, int x, int y) {
+        if (x < 0 || x >= m || y < 0 || y >= n || grid[x][y] == '0') {
+            return;
+        }
+        grid[x][y] = '0';
+
+        dfs(grid, x + 1, y);
+        dfs(grid, x - 1, y);
+        dfs(grid, x, y + 1);
+        dfs(grid, x, y - 1);
+    }
+    
+    int numIslands(vector<vector<char>>& grid) {
+        m = grid.size();
+        n = grid[0].size();
+        int count = 0;
+
+        for(int i = 0; i < m; i++) {
+            for(int j = 0; j < n; j++) {
+                if (grid[i][j] == '1') {
+                    count++;
+                    dfs(grid, i, j);
+                }
+            }
+        }
+        return count;
+    }
+};