solve: reorder list

Author: sounmind

reorder-list/evan.js

@@ -0,0 +1,77 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {void} Do not return anything, modify head in-place instead.
+ */
+var reorderList = function (head) {
+  const middleNode = findMiddleNode(head);
+  let reversedHalf = reverseList(middleNode.next);
+  middleNode.next = null;
+
+  let half = head;
+
+  while (reversedHalf) {
+    const nextForward = half.next;
+    const nextBackward = reversedHalf.next;
+
+    half.next = reversedHalf;
+    reversedHalf.next = nextForward;
+
+    reversedHalf = nextBackward;
+    half = nextForward;
+  }
+};
+/**
+ * Time Complexity: O(n)
+ * - Finding the middle node takes O(n).
+ * - Reversing the second half takes O(n).
+ * - Merging the two halves takes O(n).
+ * - Overall, the time complexity is O(n) + O(n) + O(n) = O(n).
+ *
+ * Space Complexity: O(1)
+ * - We only use a constant amount of extra space (pointers),
+ *   so the space complexity is O(1).
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var reverseList = function (head) {
+  let currentNode = head;
+  let previousNode = null;
+
+  while (currentNode !== null) {
+    const nextNode = currentNode.next;
+
+    currentNode.next = previousNode;
+    previousNode = currentNode;
+    currentNode = nextNode;
+  }
+
+  return previousNode;
+};
+
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var findMiddleNode = function (head) {
+  if (!head) return null;
+
+  let slow = head;
+  let fast = head;
+
+  while (fast !== null && fast.next !== null) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  return slow;
+};