Add week 11 solutions: reorder-list

Author: gitsunmin

reorder-list/gitsunmin.ts

@@ -0,0 +1,53 @@
+/**
+ * https://leetcode.com/problems/reorder-list/
+ * time complexity : O(n)
+ * space complexity : O(1)
+ */
+
+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)
+    }
+}
+
+function reorderList(head: ListNode | null): void {
+    if (!head || !head.next) return;
+
+    // 1. 중간 지점 찾기 (Floyd’s Tortoise and Hare)
+    let slow: ListNode | null = head;
+    let fast: ListNode | null = head;
+
+    while (fast && fast.next) {
+        slow = slow!.next;
+        fast = fast.next.next;
+    }
+
+    // 2. 중간 이후의 리스트 뒤집기
+    let prev: ListNode | null = null;
+    let curr: ListNode | null = slow;
+
+    while (curr) {
+        const next = curr.next;
+        curr.next = prev;
+        prev = curr;
+        curr = next;
+    }
+
+    // 3. 앞부분과 뒤집어진 후반부 병합
+    let first: ListNode | null = head;
+    let second: ListNode | null = prev;
+
+    while (second && second.next) {
+        const temp1 = first!.next;
+        const temp2 = second.next;
+
+        first!.next = second;
+        second.next = temp1;
+
+        first = temp1;
+        second = temp2;
+    }
+};