8. Math

1. Fizz Buzz

https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/743/

if-else:

class Solution {
public:
    vector<string> fizzBuzz(int n) {
        vector<string> result;
        for(int i = 1; i <= n; i++){
            if((i % 3 == 0) && (i % 5 == 0)){
                result.push_back("FizzBuzz");
            } else if((i % 3 == 0) && (i % 5 != 0)){
                result.push_back("Fizz");
            } else if((i % 3 != 0) && (i % 5 == 0)){
                result.push_back("Buzz");
            } else {
                result.push_back(to_string(i));
            }
        }
        return result;
    }
};

2. Count Primes

https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/744/

write a isPrime() approach:

class Solution {
public:
    int countPrimes(int n) {
        int count = 0;
        for(int i = 2; i < n; i++){ //the requirement is "less than n", not including itself
            if(isPrime(i)){
                count++;
            }
        }
        return count;
    }
private:
    bool isPrime(int n) {
        if(n <= 1)
            return false;
        for(int i = 2; i * i <= n; i++){
            if(n % i == 0)
                return false;
        }
        return true;
    }
};

3. Power of Three

https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/745/

recursively check powers of 3:

• simple logic but a bit complicated code

• skip this

  check the remainder of fmod:

  class Solution {
  public:
    bool isPowerOfThree(int n) {
        return fmod(log10(n)/log10(3), 1) == 0;
    }
  };

• fmod() has 2 parameters, simply it returns a float value, in this case is:

  • % 1
  • is a float value here
• use instead of is because
  • 
  • so
  • it’s better for C++ to just apply log10() to finish it

4. Roman to Integer

https://leetcode.com/explore/interview/card/top-interview-questions-easy/102/math/878/

switch and go over:

class Solution {
public:
    int roman_map(char s){
        switch(s) {
            case 'I': return 1;
            case 'V': return 5;
            case 'X': return 10;
            case 'L': return 50;
            case 'C': return 100;
            case 'D': return 500;
            case 'M': return 1000;
            default: return 0;
        }
    }
    int romanToInt(string s) {
        int sum = 0;
        for(int i = 0; i < s.size(); i++){
            if(i>0 && roman_map(s[i]) > roman_map(s[i-1]))
                sum += (roman_map(s[i]) - 2 * roman_map(s[i-1])); //need to subtract the previous one
            else
                sum += roman_map(s[i]);
        }
        return sum;
    }
};+