102. Binary Tree Level Order Traversal (#98)

Author: Kohei Asai

solutions/binaryTreeLevelOrderTraversal.test.ts

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+import { createBinaryTreeNode } from "../utilities/TreeNode";
+import levelOrder from "./binaryTreeLevelOrderTraversal";
+
+describe("102. Binary Tree Level Order Traversal", () => {
+  const TEST_CASES = new Map([
+    [[1, null, 2, 3], [[1], [2], [3]]],
+    [[4, 4, 5, 1, 2], [[4], [4, 5], [1, 2]]],
+    [
+      [1, null, 1, null, 1, null, 1, null, 1, null, 1, 2],
+      [[1], [1], [1], [1], [1], [1], [2]]
+    ],
+    [[1], [[1]]],
+    [[], []]
+  ]);
+
+  for (const [tree, expected] of TEST_CASES) {
+    it(`returns [${expected.map(
+      level => `[${level}]`
+    )}] when called with [${tree}]`, () => {
+      expect(levelOrder(createBinaryTreeNode(tree))).toEqual(expected);
+    });
+  }
+});

solutions/binaryTreeLevelOrderTraversal.ts

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+import { TreeNode } from "../utilities/TreeNode";
+
+// 102. Binary Tree Level Order Traversal
+// https://leetcode.com/problems/binary-tree-level-order-traversal/
+export default function levelOrder<T>(root: TreeNode<T> | null): T[][] {
+  const valuesEachLevel: T[][] = [];
+
+  function traverse(node: TreeNode<T> | null, level: number) {
+    if (node === null) return;
+
+    if (valuesEachLevel[level]) {
+      valuesEachLevel[level].push(node.val);
+    } else {
+      valuesEachLevel[level] = [node.val];
+    }
+
+    traverse(node.left, level + 1);
+    traverse(node.right, level + 1);
+  }
+
+  traverse(root, 0);
+
+  return valuesEachLevel;
+}