reorder list solution

Author: hyer0705

reorder-list/hyer0705.ts

@@ -0,0 +1,56 @@
+/**
+ * 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)
+ *     }
+ * }
+ */
+
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+/**
+ Do not return anything, modify head in-place instead.
+ */
+function reorderList(head: ListNode | null): void {
+  if (!head || !head.next || !head.next.next) return;
+
+  let slow: ListNode | null = head;
+  let fast: ListNode | null = head;
+
+  // find central node
+  while (fast && fast.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  let secondHalf = slow.next;
+  slow.next = null;
+
+  // reverse secondHalf
+  let prev = null;
+  let curr = secondHalf;
+  while (curr) {
+    const nextTemp = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = nextTemp;
+  }
+
+  let first = head;
+  let second = prev;
+
+  while (second) {
+    const firstNext = first.next;
+    const secondNext = second.next;
+
+    first.next = second;
+    second.next = firstNext;
+
+    first = firstNext;
+    second = secondNext;
+  }
+}