reorder list solution

Author: byol-han

reorder-list/byol-han.js

@@ -0,0 +1,48 @@
+/**
+ * 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) {
+  if (!head || !head.next) return;
+
+  // 1. 중간 지점 찾기 (slow, fast 포인터 사용)
+  let slow = head;
+  let fast = head;
+  while (fast.next && fast.next.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  // 2. 중간 이후 리스트 뒤집기
+  let prev = null;
+  let curr = slow.next;
+  while (curr) {
+    let nextTemp = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = nextTemp;
+  }
+  // 중간 지점 이후는 끊기
+  slow.next = null;
+
+  // 3. 앞쪽 리스트와 뒤쪽 리스트 교차 병합
+  let first = head;
+  let second = prev;
+  while (second) {
+    let tmp1 = first.next;
+    let tmp2 = second.next;
+
+    first.next = second;
+    second.next = tmp1;
+
+    first = tmp1;
+    second = tmp2;
+  }
+};