Added levelOrder solution

Author: nhistory

binary-tree-level-order-traversal/nhistory.js

@@ -0,0 +1,32 @@
+var levelOrder = function (root) {
+  // Edge case: If root is null, return []
+  if (root === null) return [];
+
+  // Create result and queue
+  let result = [];
+  let queue = [root];
+
+  // Iterate while queue.length is exist
+  while (queue.length) {
+    // Create levelArr and levelSize
+    let levelArr = [];
+    let levelSize = queue.length;
+    // Initiate currentNode from queue by using shift method
+    while (levelSize) {
+      const currentNode = queue.shift();
+      levelArr.push(currentNode.val);
+      // If currentNode.left is not null, push into the queue
+      if (currentNode.left) queue.push(currentNode.left);
+      // If currentNode.right is not null, push into the queue
+      if (currentNode.right) queue.push(currentNode.right);
+
+      levelSize--;
+    }
+    // Push levelArr into result
+    result.push(levelArr);
+  }
+  return result;
+};
+
+// TC: O(n)
+// SC: O(n)