148. Sort List

链表排序，使用归并排序

• Runtime: 268 ms, faster than 17.27%
• Memory Usage: 64.9 MB, less than 5.18%

/**
 * 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 {ListNode}
 */
var sortList = function(head) {
    if(!head || !head.next) return head
    let fast = head
    let slow = head
    let prev = null
    while(fast && fast.next) {
        prev = slow
        fast = fast.next.next
        slow = slow.next
    }
    prev.next = null
    let l1 = sortList(head)
    let l2 = sortList(slow)
    
    return mergeTwoLists(l1, l2)
    
};
var mergeTwoLists = function(l1, l2) {
    let dummy = new ListNode(0)
    let res = dummy
    while(l1 && l2) {
        if(l1.val < l2.val) {
            res.next = new ListNode(l1.val)
            l1 = l1.next
        } else {
            res.next = new ListNode(l2.val)
            l2 = l2.next
        }
        res = res.next
    }
    if (l1) {
        res.next = l1
    } else {
        res.next = l2
    }
    return dummy.next
};