leetcode 338. Counting Bits

Posted 2020-06-27 Dystopia

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

• class Solution {
  
  public:
      vector<int> countBits(int num) {
          vector<int> count_v(num+1,0);
          for(int i = 1; i <= num;i++)
          {
              int x = i;
              int count=0;
              while(x)
              {
                  if((x & 1)==1) count++;
                  x >>= 1;
              }
              count_v[i] = count;
          }
          return count_v;
      }
  
  };

  注意：1.计算二进制中1的个数，难点在于时间复杂度。有三种基本求二进制1个数的方法：

• （1）定理求解：取余数的方法，最慢。
• （2）基本法（雾）：值和1进行&操作，然后>>算术右移，仍然有两个循环。（即为本道题我的解法）
• （3）快速法：n和n-1进行&操作，可以消去二进制最右的1，仍然有两个循环。（在191题中的解法）

• 参考：http://www.cnblogs.com/graphics/archive/2010/06/21/1752421.html