338. Counting Bits 和191. Number of 1 Bits

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第一题、338. Counting Bits
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.
方法一、i&(i-1)可以消除最后一个1的特性，代码如下：

vector<int> countBits(int num) {vector<int> ret(num+1,0);for(int i = 1; i <= num; i++)ret[i] = ret[i&(i-1)] + 1; //由于i&(i-1)可以消除最后一个1，所以可以这样做return ret;}

方法二、普通的做法:

vector<int> countBits(int num) {vector<int> total(num+1);for(int i=0;i<=num;i++){total[i]=0;int j=i;while(j){j&=(j-1);total[i]++;}}return total;}

第二题、191. Number of 1 Bits
Write a function that takes an unsigned integer and returns the number of ’1’ bits it has (also known as the Hamming weight).

For example, the 32-bit integer ’11’ has binary representation 00000000000000000000000000001011, so the function should return 3
题意：统计一个整数的32位的二进制中1的个数，和上题的思路类似

int hammingWeight(uint32_t n) {int count = 0;while(n){n &= (n -1);count++;}return count;}

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