feat: $200 增加Java Code

problems/200.number-of-islands.md

@@ -53,7 +53,37 @@ Output: 3
 
 ## 代码
 
-* 语言支持：JS, python3
+* 语言支持：JS, python3，Java
+
+Java Code：
+
+```java
+   public int numIslands(char[][] grid) {
+        if (grid == null || grid.length == 0 || grid[0].length == 0) return 0;
+
+        int count = 0;
+        for (int row = 0; row < grid.length; row++) {
+            for (int col = 0; col < grid[0].length; col++) {
+                if (grid[row][col] == '1') {
+                    dfs(grid, row, col);
+                    count++;
+                }
+            }
+        }
+        return count;
+    }
+
+    private void dfs(char[][] grid,int row,int col) {
+        if (row<0||row== grid.length||col<0||col==grid[0].length||grid[row][col]!='1') {
+            return;
+        }
+        grid[row][col] = '0';
+        dfs(grid, row-1, col);
+        dfs(grid, row+1, col);
+        dfs(grid, row, col+1);
+        dfs(grid, row, col-1);
+    }
+```
 
 Javascript Code:
 ```js