Merge K sorted Lists@leetcode

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Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.
两种办法，第一种很straight forward， 直接merge two sorted lists， 然后对于每一个lists，都如此合并，但时间复杂度消耗比较高。e.g, n lists, every list has k nodes. O(knn). 因为每次合并之后的lists长度都会变长。
第二种用min-heap来动态地给minimun node in all lists. time complexity is O(knlogk). (介个我不太会。。因为没咋建过堆)
第三种是我最稀饭的，用multiset来动态地保存当前最小的元素，然后比较完一个就把那个list的后面那个扔到multiset里面去。

Multisets are containers that store elements following a specific order, and where multiple elements can have equivalent values.

In a multiset, the value of an element also identifies it (the value is itself the key, of type T). The value of the elements in a multiset cannot be modified once in the container (the elements are always const), but they can be inserted or removed from the container.

Internally, the elements in a multiset are always sorted following a specific strict weak ordering criterion indicated by its internal comparison object (of type Compare).

multiset containers are generally slower than unordered_multiset containers to access individual elements by their key, but they allow the direct iteration on subsets based on their order.

Multisets are typically implemented as binary search trees.