finish 143

Author: BaffinLee

101-200/143. Reorder List.md

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+# 143. Reorder List
+
+- Difficulty: Medium.
+- Related Topics: Linked List.
+- Similar Questions: .
+
+## Problem
+
+Given a singly linked list **L**: **L**0→**L**1→…→**L****n**-1→**L**n,
+reorder it to: **L**0→**L****n**→**L**1→**L****n**-1→**L**2→**L****n**-2→…
+
+You may **not** modify the values in the list's nodes, only nodes itself may be changed.
+
+**Example 1:**
+
+```
+Given 1->2->3->4, reorder it to 1->4->2->3.
+```
+
+**Example 2:**
+
+```
+Given 1->2->3->4->5, reorder it to 1->5->2->4->3.
+```
+
+## Solution
+
+```javascript
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {void} Do not return anything, modify head in-place instead.
+ */
+var reorderList = function(head) {
+  if (!head || !head.next || !head.next.next) return;
+  
+  // find mid
+  var mid = null;
+  var fast = head;
+  var slow = head;
+  while (fast.next && fast.next.next && slow.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+  mid = slow;
+  
+  // reverse the later part
+  var now = mid.next.next;
+  var second = mid.next;
+  var tmp = null;
+  second.next = null;
+  while (now) {
+    tmp = now.next;
+    now.next = second;
+    second = now;
+    now = tmp;
+  }
+  mid.next = second;
+  
+  // insert one after another
+  var before = head;
+  var after = mid.next;
+  mid.next = null;
+  while (after) {
+    tmp = before.next;
+    before.next = after;
+    before = tmp;
+    tmp = after.next;
+    after.next = before;
+    after = tmp
+  }
+};
+```
+
+**Explain:**
+
+比如 `1->2->3->4->5->6` ，分三步
+
+1. 找到 `mid = 3`
+2. 翻转后半部分，变成 `1->2->3->6->5->4`
+3. 后半部分依次插入到前半部分
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(1).