重排链表

1.重排链表/复习.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) {
+  let slow = head
+  let fast = head
+
+  // 使用快慢指针寻找链表中点
+  while (fast.next && fast.next.next) {
+    fast = fast.next.next
+    slow = slow.next
+  }
+
+  // 拆分成前后链表
+  let cur = slow.next
+  slow.next = null
+
+  // 反转后链表
+  let pre = null
+  while (cur) {
+    let next = cur.next // 4 null
+    cur.next = pre // null 3
+    pre = cur // 3 4
+    cur = next // 4 null
+    // null <- 3 <- 4
+  }
+
+  // 合并前后链表
+  let lp = head
+  while (lp && pre) {
+    let ln = lp.next
+    let rn = pre.next
+    lp.next = pre
+    pre.next = ln
+    lp = ln
+    pre = rn
+  }
+
+  return head
+};