K 路合并算法

用归并实现 k 路合并:

class Solution:
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        # 归并
        if len(lists) == 0: return None
        def merge(start, end):
            if end == start:
                return lists[start]
            mid = start + (end - start) // 2
            left = merge(start, mid)
            right = merge(mid + 1, end)
            return self.mergeTwoLists(left, right)

        return merge(0, len(lists) - 1)
    def mergeTwoLists(self, l1, l2):
        pointer = helper = ListNode()
        while l1 and l2:
            if l1.val <= l2.val:
                pointer.next = l1
                l1 = l1.next
            else:
                pointer.next = l2
                l2 = l2.next
            pointer = pointer.next
        if l1:
            pointer.next = l1
        else:
            pointer.next = l2

        return helper.next