Math

• Happy number https://leetcode.com/problems/happy-number/description/

int digitSquareSum(int n) {
    int sum = 0, tmp;
    while (n) {
        tmp = n % 10;
        sum += tmp * tmp;
        n /= 10;
    }
    return sum;
}

bool isHappy(int n) {
    int slow, fast;
    slow = fast = n;
    do {
        slow = digitSquareSum(slow);
        fast = digitSquareSum(fast);
        fast = digitSquareSum(fast);
    } while(slow != fast);
    if (slow == 1) return 1;
    else return 0;
}

• Count primes

class Solution {
public:
    int countPrimes(int n) {
        vector<bool> prime(n, true);
        prime[0] = false, prime[1] = false;
        for (int i = 0; i < sqrt(n); ++i) {
            if (prime[i]) {
                for (int j = i*i; j < n; j += i) {
                    prime[j] = false;
                }    
            }    
        }
        return count(prime.begin(), prime.end(), true);
    }
};

• Next Permutation

class Solution {
public:
    void nextPermutation(vector<int> &num) 
    {
        if (num.empty()) return;

        // in reverse order, find the first number which is in increasing trend (we call it violated number here)
        int i;
        for (i = num.size()-2; i >= 0; --i)
        {
            if (num[i] < num[i+1]) break;
        }

        // reverse all the numbers after violated number
        reverse(begin(num)+i+1, end(num));
        // if violated number not found, because we have reversed the whole array, then we are done!
        if (i == -1) return;
        // else binary search find the first number larger than the violated number
        auto itr = upper_bound(begin(num)+i+1, end(num), num[i]);
        // swap them, done!
        swap(num[i], *itr);
    }
};