foolish fly fox's blog

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--Forever young, forever weep.

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338. Counting Bits

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

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.

Solution

class Solution:
    def countBits(self, num):
        """
        :type num: int
        :rtype: List[int]
        """
        results = [0]*(num+1)
        i = 1
        while i < num+1:
            for j in range(i, min(i*2, num+1)):
                results[j] = results[j-i]+1
            i *= 2
        return results