一道面试题

How to find the most frequent number in a sequence of number?  Any ideas?

hashtable average o(n) worst o(n^2)      sort + traverse o(nlogn)

用一个hashmap把每个数字出现的次数都统计出来，同时维护一个变量m，记录当前出现次数最多的元素。每当一个数字n新出现一次，就看是否n目前出现的次数是否超过m的次数，如果超过，就把m替换为n。然后重复这个过程直到检查完sequence中每个元素即可。把变量m替换为一个大小为k的heap（或者binary search tree），就能够实现找出前k个最常见的元素。

the easiest method I can come up with is also using a hashtable to store keys and values.
Any limitations? Time complexity? Space? If it requires O(1) space, I don't know then...

there is an O(N) algorithm