23. Merge k Sorted Lists

Description

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

 

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

 

Constraints:

• k == lists.length
• 0 <= k <= 104
• 0 <= lists[i].length <= 500
• -104 <= lists[i][j] <= 104
• lists[i] is sorted in ascending order.
• The sum of lists[i].length will not exceed 104.

 

Solutions

Solution: Hash Map

• Time complexity: O(2n)
• Space complexity: O(n)

 

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
const mergeKLists = function (lists) {
  const list = new ListNode();
  const hashMap = new Map();
  let current = list;
  let maxVal = 0;
  let minVal = Number.MAX_SAFE_INTEGER;

  for (let node of lists) {
    while (node) {
      const { val, next } = node;
      const count = hashMap.get(val) ?? 0;

      hashMap.set(val, count + 1);
      node = next;
      maxVal = Math.max(val, maxVal);
      minVal = Math.min(val, minVal);
    }
  }
  for (let val = minVal; val <= maxVal; val++) {
    if (!hashMap.has(val)) continue;
    let count = hashMap.get(val);

    while (count--) {
      current.next = new ListNode(val);
      current = current.next;
    }
  }
  return list.next;
};