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.
1、二进制方法
class Solution { public: 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; //i&(i-1)减掉了一个二进制1 return ret; } };
2、模板库方法
class Solution { public: vector<int> countBits(int num) { vector<int> vec; for(int i=0;i<=num;++i) { bitset<32> bs(i); vec.push_back(bs.count()); } return vec; } };
作者:弦断
出处:http://www.cnblogs.com/ucas/
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