题目
counting-bits
2. 算法
* 直接模拟
* bitset
* 技巧
* 位操作
3. 代码
* 直接模拟
class Solution {
public:
vector<int> countBits(int num) {
if (num == 0) return {0};
vector<int> res{0, 1};
int k = 2, i = 2;
while (i <= num) {
for (i = pow(2, k - 1); i < pow(2, k); ++i) {
if (i > num) break;
int t = (pow(2, k) - pow(2, k - 1)) / 2;
if (i < pow(2, k - 1) + t) res.push_back(res[i - t]);
else res.push_back(res[i - t] + 1);
}
++k;
}
return res;
}
};
* bitset
class Solution {
public:
vector<int> countBits(int num) {
vector<int> res;
for (int i = 0; i <= num; ++i) {
res.push_back(bitset<32>(i).count());
}
return res;
}
};
* 技巧
class Solution {
public:
vector<int> countBits(int num) {
vector<int> res{0};
for (int i = 1; i <= num; ++i) {
if (i % 2 == 0) res.push_back(res[i / 2]);
else res.push_back(res[i / 2] + 1);
}
return res;
}
};
* 位操作
class Solution {
public:
vector<int> countBits(int num) {
vector<int> res(num + 1, 0);
for (int i = 1; i <= num; ++i) {
res[i] = res[i & (i - 1)] + 1;
}
return res;
}
};