LeetCode #827: Making A Large Island

Author: ISKU

LeetCode/827/Solution.java

@@ -0,0 +1,102 @@
+/*
+ * Author: Minho Kim (ISKU)
+ * Date: November 14, 2019
+ * E-mail: minho.kim093@gmail.com
+ *
+ * https://github.com/ISKU/Algorithm
+ * https://leetcode.com/problems/making-a-large-island/
+ */
+
+class Solution {
+
+	private final int[] dy = { 1, -1, 0, 0 };
+	private final int[] dx = { 0, 0, 1, -1 };
+
+	private int[][] map;
+	private boolean[][] visited;
+
+	private Map<Point, Integer> groupMap;
+	private int[] count;
+	private int Y, X, answer;
+
+	public int largestIsland(int[][] grid) {
+		map = grid;
+		Y = map.length;
+		X = map[0].length;
+		groupMap = new HashMap<>();
+
+		int groupNumber = 0;
+		count = new int[2500];
+		visited = new boolean[Y][X];
+		for (int y = 0; y < Y; y++)
+			for (int x = 0; x < X; x++)
+				if (!visited[y][x] && map[y][x] == 1)
+					dfs(y, x, groupNumber++);
+
+		for (int y = 0; y < Y; y++) {
+			for (int x = 0; x < X; x++) {
+				if (map[y][x] == 1)
+					continue;
+
+				Set<Integer> set = new HashSet<>();
+				for (int i = 0; i < 4; i++) {
+					int ny = y + dy[i];
+					int nx = x + dx[i];
+					if (ny < 0 || ny >= Y || nx < 0 || nx >= X || map[ny][nx] == 0)
+						continue;
+
+					Point p = new Point(ny, nx);
+					int group = groupMap.get(p);
+					set.add(group);
+				}
+
+				int sum = 0;
+				if (!set.isEmpty())
+					for (int group : set)
+						sum += count[group];
+
+				answer = Math.max(answer, sum + 1);
+			}
+		}
+
+		return answer;
+	}
+
+	private void dfs(int y, int x, int group) {
+		visited[y][x] = true;
+		groupMap.put(new Point(y, x), group);
+		count[group]++;
+		answer = Math.max(answer, count[group]);
+
+		for (int i = 0; i < 4; i++) {
+			int ny = y + dy[i];
+			int nx = x + dx[i];
+			if (ny < 0 || ny >= Y || nx < 0 || nx >= X || visited[ny][nx] || map[ny][nx] == 0)
+				continue;
+
+			dfs(ny, nx, group);
+		}
+	}
+
+	private class Point {
+		public int y, x;
+
+		public Point(int y, int x) {
+			this.y = y;
+			this.x = x;
+		}
+
+		@Override
+		public int hashCode() {
+			return y * 2500 + x;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			Point p = (Point) o;
+			if (this.y == p.y && this.x == p.x)
+				return true;
+			return false;
+		}
+	}
+}