From c230f110f71b326b6c358e386fea87206c6e6642 Mon Sep 17 00:00:00 2001
From: Kohei Asai <me@axross.io>
Date: Wed, 25 Sep 2019 18:32:53 -0700
Subject: [PATCH] 145. Binary Tree Postorder Traversal

---
 .../binaryTreePostorderTraversal.test.ts      | 21 +++++++++++++++++++
 solutions/binaryTreePostorderTraversal.ts     | 20 ++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 solutions/binaryTreePostorderTraversal.test.ts
 create mode 100644 solutions/binaryTreePostorderTraversal.ts

diff --git a/solutions/binaryTreePostorderTraversal.test.ts b/solutions/binaryTreePostorderTraversal.test.ts
new file mode 100644
index 0000000..f84e0e0
--- /dev/null
+++ b/solutions/binaryTreePostorderTraversal.test.ts
@@ -0,0 +1,21 @@
+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);
+    });
+  }
+});
diff --git a/solutions/binaryTreePostorderTraversal.ts b/solutions/binaryTreePostorderTraversal.ts
new file mode 100644
index 0000000..4c63563
--- /dev/null
+++ b/solutions/binaryTreePostorderTraversal.ts
@@ -0,0 +1,20 @@
+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;
+}