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0827-making-a-large-island/0827-making-a-large-island.java

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+class Solution {
+    private int n;
+    private int grid[][];
+    private Map<Integer, Integer> map = new HashMap<>();
+    private static final int directions[][] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
+
+    public int largestIsland(int[][] grid) {
+        this.n = grid.length;
+        this.grid = grid;
+        int color = 2;
+        int area = 0;
+        boolean hasZero = false;
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                if (grid[i][j] == 1) {
+                    int size = dfs(i, j, color);
+                    map.put(color, size);
+                    area = Math.max(area, size);
+                    color++;
+                } 
+                else hasZero = true;
+            }
+        }
+
+        if (!hasZero) return area;
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                if (grid[i][j] == 0) {
+                    Set<Integer> set = new HashSet<>();
+                    int curr = 1;
+
+                    for (int dir[] : directions) {
+                        int x = i + dir[0], y = j + dir[1];
+                        if (isValid(x, y) && grid[x][y] > 1 && set.add(grid[x][y])) {
+                            curr += map.get(grid[x][y]);
+                        }
+                    }
+
+                    area = Math.max(area, curr);
+                }
+            }
+        }
+
+        return area;
+    }
+
+    private int dfs(int i, int j, int color) {
+        if (!isValid(i, j) || grid[i][j] != 1) return 0;
+
+        grid[i][j] = color;
+        int size = 1;
+
+        for (int dir[] : directions) {
+            size += dfs(i + dir[0], j + dir[1], color);
+        }
+
+        return size;
+    }
+
+    private boolean isValid(int i, int j) {
+        return i >= 0 && i < n && j >= 0 && j < n;
+    }
+}