Added numIslands solution

Author: nhistory

number-of-islands/nhistory.js

@@ -0,0 +1,42 @@
+var numIslands = function (grid) {
+  // Declare row and column length
+  const m = grid.length,
+    n = grid[0].length;
+  let numIslands = 0;
+
+  // Available directions for depth-first search
+  const dir = [
+    [0, 1],
+    [1, 0],
+    [0, -1],
+    [-1, 0],
+  ];
+
+  // Function to depth-first search inside of grid
+  const dfs = (i, j) => {
+    grid[i][j] = "2";
+
+    let x, y;
+    for (d of dir) {
+      x = i + d[0];
+      y = j + d[1];
+      if (x >= 0 && x < m && y >= 0 && y < n && grid[x][y] === "1") {
+        dfs(x, y);
+      }
+    }
+    return;
+  };
+
+  for (let i = 0; i < m; i++) {
+    for (let j = 0; j < n; j++) {
+      if (grid[i][j] === "1") {
+        dfs(i, j);
+        numIslands++;
+      }
+    }
+  }
+  return numIslands;
+};
+
+// TC: O(m*n)
+// SC: O(m*n)