Adding bottom view binary tree problem solution

- Solution based on https://www.geeksforgeeks.org/bottom-view-binary-tree/
- Modified `.gitignore` to ignore Visual Studio's files

Author: karimelazzouni

.gitignore

@@ -1,2 +1,3 @@
 node_modules/
-coverage/
+coverage/
+.vs/

package-lock.json

@@ -20,5 +20,8 @@
     "eslint-config-airbnb-base": "^13.1.0",
     "eslint-plugin-import": "^2.14.0",
     "jest": "^25.0.0"
+  },
+  "dependencies": {
+    "hashmap": "^2.4.0"
   }
 }

package.json

@@ -0,0 +1,40 @@
+const HashMap = require('hashmap');
+const Queue = require('../../../Queue');
+
+// Determines the bottom view of a binary tree
+// Takes a BinaryTree as a parameter
+// Returns an integer array
+// Time complexity: O(n) where n is the number of nodes in the tree
+
+module.exports = function bottomView(binaryTree) {
+  if (binaryTree == null || binaryTree.root == null) {
+    return [];
+  }
+
+  // root's horizontal distance = 0
+  const horizontalDistance = 0;
+
+  // create a map to track most recent visited nodes per hd
+  const hdToNodeValue = new HashMap();
+
+  // perform bfs
+  const q = new Queue();
+  q.enqueue([binaryTree.root, horizontalDistance]);
+
+  while (q.length() > 0) {
+    const currentNodeTuple = q.dequeue();
+    const currentNode = currentNodeTuple[0];
+    const currentHd = currentNodeTuple[1];
+    hdToNodeValue.set(currentHd, currentNode.value);
+
+    if (currentNode.leftChild != null && currentNode.leftChild.value != null) {
+      q.enqueue([currentNode.leftChild, currentHd - 1]);
+    }
+
+    if (currentNode.rightChild != null && currentNode.rightChild.value != null) {
+      q.enqueue([currentNode.rightChild, currentHd + 1]);
+    }
+  }
+
+  return hdToNodeValue.values();
+};