up solution and test of problem middle of linkedlist

Author: sangtrandev00

src/leetcode/linked_list/middle-of-linked-list.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, test } from 'bun:test';
+import { ListNode, middleNode } from './middle-of-linked-list';
+
+describe('middleNode', () => {
+    test('should return null for an empty list', () => {
+        expect(middleNode(null)).toBeNull();
+    });
+
+    test('should return the only node for a single-node list', () => {
+        const head = new ListNode(1);
+        expect(middleNode(head)).toBe(head);
+    });
+
+    test('should return the middle node for an odd-length list', () => {
+        const head = new ListNode(1, new ListNode(2, new ListNode(3)));
+        expect(middleNode(head)).toBe(head.next); // Middle node is 2
+    });
+
+    test('should return the second middle node for an even-length list', () => {
+        const head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4))));
+        expect(middleNode(head)).toBe(head?.next?.next ?? null); // Middle node is 3
+    });
+
+    test('should return the middle node for a longer odd-length list', () => {
+        const head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
+        expect(middleNode(head)).toBe(head?.next?.next ?? null); // Middle node is 3
+    });
+
+    test('should return the second middle node for a longer even-length list', () => {
+        const head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5, new ListNode(6))))));
+        expect(middleNode(head)).toBe(head?.next?.next?.next ?? null); // Middle node is 4
+    });
+});

src/leetcode/linked_list/middle-of-linked-list.ts

@@ -0,0 +1,36 @@
+/**
+ * Definition for singly-linked list.
+ * class ListNode {
+ *     val: number
+ *     next: ListNode | null
+ *     constructor(val?: number, next?: ListNode | null) {
+ *         this.val = (val===undefined ? 0 : val)
+ *         this.next = (next===undefined ? null : next)
+ *     }
+ * }
+ */
+
+export class ListNode {
+    val: number
+    next: ListNode | null
+    constructor(val?: number, next?: ListNode | null) {
+        this.val = (val === undefined ? 0 : val)
+        this.next = (next === undefined ? null : next)
+    }
+}
+
+// Sử dụng kỹ thuật brute force O(1) and 
+// Using two pointers technique
+export function middleNode(head: ListNode | null): ListNode | null {
+    let fast = head; // O(1)
+    let slow = head; // O(1)
+    while (fast?.next) {
+        slow = slow?.next ?? null;
+        fast = fast?.next?.next ?? null
+    }
+    return slow;
+}; // Space: O(1), time (O(n/2))
+
+// Space complexity O(1) Tại vì fast và slow chỉ lưu bộ nhớ!
+// console.log("middleNode", middleNode(new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))))));
+console.log("middleNode", middleNode(new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5, new ListNode(6))))))));