sort
find mid point 
take one from the end of the two lists one by one 
class Solution(object):
    def wiggleSort(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        temp=nums[:]
        temp.sort()
        small,large=(len(nums)-1)/2,len(nums)-1
        for i in xrange(len(nums)):
            if i%2==0:
                nums[i]=temp[small]
                small-=1
            else:
                nums[i]=temp[large]
                large-=1

O(n)+O(1) after median --- Virtual Indexing

https://discuss.leetcode.com/topic/32929/o-n-o-1-after-median-virtual-indexing

read and come back to it