Feat: 143. Reorder List

Author: HC-kang

reorder-list/HC-kang.ts

@@ -0,0 +1,47 @@
+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;
+  }
+}
+
+/**
+ * https://leetcode.com/problems/reorder-list
+ * T.C. O(n)
+ * S.C. O(1)
+ */
+function reorderList(head: ListNode | null): void {
+  if (!head || !head.next) return;
+
+  let fast: ListNode | null = head;
+  let slow: ListNode | null = head;
+
+  while (fast && fast.next) {
+    fast = fast.next.next;
+    slow = slow!.next;
+  }
+
+  let prev: ListNode | null = null;
+  let curr: ListNode | null = slow;
+  while (curr) {
+    const next = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = next;
+  }
+
+  let front: ListNode | null = head;
+  let back: ListNode | null = prev;
+  while (back!.next) {
+    const frontNext = front!.next;
+    const backNext = back!.next;
+
+    front!.next = back;
+    back!.next = frontNext;
+
+    front = frontNext;
+    back = backNext;
+  }
+}