3. Number of Islands

Author: Sunjae95

number-of-islands/sunjae95.js

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+/**
+ * @description
+ * brainstorming:
+ * hash table + two pointer
+ *
+ * n = length of grid
+ * k = length of grid[index]
+ * time complexity: O(n * k)
+ * space complexity: O(n * k)
+ */
+var numIslands = function (grid) {
+  let answer = 0;
+  const visited = Array.from({ length: grid.length }, (_, i) =>
+    Array.from({ length: grid[i].length }, () => false)
+  );
+
+  const dfs = (r, c) => {
+    const dr = [0, 1, 0, -1];
+    const dc = [1, 0, -1, 0];
+
+    for (let i = 0; i < 4; i++) {
+      const nextR = r + dr[i];
+      const nextC = c + dc[i];
+
+      if (
+        nextR >= 0 &&
+        nextR < grid.length &&
+        nextC >= 0 &&
+        nextC < grid[r].length &&
+        grid[nextR][nextC] == 1 &&
+        !visited[nextR][nextC]
+      ) {
+        visited[nextR][nextC] = true;
+        dfs(nextR, nextC);
+      }
+    }
+  };
+
+  for (let row = 0; row < grid.length; row++) {
+    for (let column = 0; column < grid[row].length; column++) {
+      if (grid[row][column] == 1 && !visited[row][column]) {
+        visited[row][column] = true;
+        answer++;
+        dfs(row, column);
+      }
+    }
+  }
+  return answer;
+};