diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index ee4c0f9..1c2c0c7 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -153,4 +153,5 @@ mod p199_binary_tree_right_side_view;
 mod p637_average_of_levels_in_binary_tree;
 mod p530_minimum_absolute_difference_in_bst;
 mod p230_kth_smallest_element_in_a_bst;
-mod p98_validate_binary_search_tree;
\ No newline at end of file
+mod p98_validate_binary_search_tree;
+mod p200_number_of_islands;
\ No newline at end of file
diff --git a/src/problem/p200_number_of_islands.rs b/src/problem/p200_number_of_islands.rs
new file mode 100644
index 0000000..58bd113
--- /dev/null
+++ b/src/problem/p200_number_of_islands.rs
@@ -0,0 +1,61 @@
+/**
+ * [200] Number of Islands
+ */
+pub struct Solution {}
+
+
+// submission codes start here
+
+impl Solution {
+    fn is_in_grid(x: i32, y: i32, grid: &Vec<Vec<char>>) -> bool {
+        let m = grid.len() as i32;
+        let n = grid[0].len() as i32;
+
+        x >= 0 && x < m && y >= 0 && y < n
+    }
+
+    fn dfs(grid: &mut Vec<Vec<char>>, x: i32, y: i32) {
+        if !Self::is_in_grid(x, y, grid) {
+            return;
+        }
+
+        if grid[x as usize][y as usize] != '1' {
+            return;
+        }
+        grid[x as usize][y as usize] = '2';
+
+        Self::dfs(grid, x - 1, y);
+        Self::dfs(grid, x + 1, y);
+        Self::dfs(grid, x, y + 1);
+        Self::dfs(grid, x, y - 1);
+    }
+
+    pub fn num_islands(grid: Vec<Vec<char>>) -> i32 {
+        let m = grid.len();
+        let n = grid[0].len();
+        let mut result = 0;
+        let mut grid = grid;
+
+        for i in 0..m {
+            for j in 0..n {
+                if grid[i][j] == '1' {
+                    result += 1;
+                    Self::dfs(&mut grid, i as i32, j as i32);
+                } 
+            }
+        }
+
+        result
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_200() {
+    }
+}