145. Binary Tree Postorder Traversal (#97)

Author: Kohei Asai

solutions/binaryTreePostorderTraversal.test.ts

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+import { createBinaryTreeNode } from "../utilities/TreeNode";
+import postorderTraversal from "./binaryTreePostorderTraversal";
+
+describe("145. Binary Tree Postorder Traversal", () => {
+  const TEST_CASES = new Map([
+    [[1, null, 2, 3], [3, 2, 1]],
+    [[4, 4, 5, 1, 2], [1, 2, 4, 5, 4]],
+    [
+      [1, null, 1, null, 1, null, 1, null, 1, null, 1, 2],
+      [2, 1, 1, 1, 1, 1, 1]
+    ],
+    [[1], [1]],
+    [[], []]
+  ]);
+
+  for (const [tree, expected] of TEST_CASES) {
+    it(`returns [${expected}] when called with [${tree}]`, () => {
+      expect(postorderTraversal(createBinaryTreeNode(tree))).toEqual(expected);
+    });
+  }
+});

solutions/binaryTreePostorderTraversal.ts

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+import { TreeNode } from "../utilities/TreeNode";
+
+// 145. Binary Tree Postorder Traversal
+// https://leetcode.com/problems/binary-tree-postorder-traversal/
+export default function postorderTraversal<T>(root: TreeNode<T> | null): T[] {
+  const history: T[] = [];
+
+  function traverse(node: TreeNode<T> | null): void {
+    if (node === null) return;
+
+    traverse(node.left);
+    traverse(node.right);
+
+    history.push(node.val);
+  }
+
+  traverse(root);
+
+  return history;
+}