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Counting Bits

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Description

https://leetcode.com/problems/counting-bits/description/

Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1’s in their binary representation and return them as an array.

Example:
For num = 5 you should return [0,1,1,2,1,2].

Follow up:

  • It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in a single pass?

  • Space complexity should be O(n).

  • Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.

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Solution

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class Solution(object):
    def countBits(self, num):
        """
        :type num: int
        :rtype: List[int]
        """
        ret = [1]
        stack = [1]
        index = 0
        while index <= num:
            new_stack = []
            for count in stack:
                ret.append(count)
                ret.append(count+1)
                new_stack.append(count)
                new_stack.append(count+1)
                index += 1
            stack = new_stack
            
        return [0] + ret[0:num]