23.MergekSortedLists.js

/**
 * @param {ListNode[]} lists
 * @return {ListNode}
 */
//[ ListNode { val: 1, next: ListNode { val: 4, next: [ListNode] } },
//   ListNode { val: 1, next: ListNode { val: 3, next: [ListNode] } },
//   ListNode { val: 2, next: ListNode { val: 6, next: null } } ]

var mergeKLists = function(lists) {
    let mergeTwoLists = (l1,l2) => {
        let preHead = new ListNode(-1)
        let preNode = preHead
        while(l1 && l2){
            if(l1.val <= l2.val){
                preNode.next = l1
                l1 = l1.next
            }else{
                preNode.next = l2
                l2 = l2.next
            }
            preNode = preNode.next
        }
        preNode.next = l1 ? l1 : l2
        return preHead.next
    }
    let n = lists.length
    if(n == 0) return null
    let res = lists[0]
    for(let i = 1;i < n;i++){
        if(lists[i]){
            res = mergeTwoLists(res,lists[i])
        }
    }
    return res
};