23.合并k个排序链表.js

/*
 * @lc app=leetcode.cn id=23 lang=javascript
 *
 * [23] 合并K个排序链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
var mergeKLists = function (lists) {
  if (lists.length == 0) {
    return null
  }
  if (lists.length == 1) {
    return lists[0];
  }
  if (lists.length == 2) {
    return merge2lists(lists[0], lists[1]);
  }
  //分治
  let mid = lists.length >> 1;
  let l1 = [];
  for (let i = 0; i < mid; i++) {
    l1[i] = lists[i];
  }
  let l2 = [];
  for (let i = mid, j = 0; i < lists.length; i++, j++) {
    l2[j] = lists[i];
  }
  return merge2lists(mergeKLists(l1), mergeKLists(l2));
};
function merge2lists(l1, l2) {
  if (!l1) {
    return l2;
  }
  if (!l2) {
    return l1;
  }
  let head = new ListNode(0);
  let node = head;
  while (l1 && l2) {
    if (l1.val < l2.val) {
      node.next = l1;
      l1 = l1.next;
    } else {
      node.next = l2;
      l2 = l2.next;
    }
    node = node.next;
  }
  node.next = l1 ? l1 : l2;
  return head.next;
}
// @lc code=end