1823 - Find the Winner of the Circular Game

 JAVA

class Solution {
    public int findTheWinner(int n, int k) {
        if (n == 1) {
            return 1;
        }
        int ans = (findTheWinner(n - 1, k) + k) % n;
        return ans == 0 ? n : ans;
    }
}

C++

class Solution {
public:
    int findTheWinner(int n, int k) {
        set<int> s;
        for (int i = 1; i <= n; ++i) {
            s.insert(i);
        }
        auto it = begin(s);
        while (s.size() > 1) {
            for (int i = 1; i < k; ++i) {
                it = it == prev(s.end()) ? begin(s) : next(it);
            }
            auto j = it == prev(s.end()) ? begin(s) : next(it);
            s.erase(it);
            it = j;
        }
        return *s.begin();
    }
};