arc117 C - Tricolor Pyramid

题意：

https://atcoder.jp/contests/arc117/tasks/arc117_c

思路：

妙

https://atcoder.jp/contests/arc117/editorial/1136

void sol() {
    init();
    int n, ans = 0; cin >> n;
    for(int i = 1; i <= n; i++) {
        char c; cin >> c; // n > p, 得用lucas
        (ans += lucas(n - 1, n - i) * (c == 'B' ? 0 : (c == 'W' ? 1 : 2))) %= 3;
    }
    ans %= 3;
    ans = ((n & 1 ? 1 : -1) * ans + 3) % 3;
    cout << (ans == 0 ? 'B' : (ans == 1 ? 'W' : 'R'));
}