LeetCode 2266. Count Number of Texts

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class Solution {
    using LL = long long;
    const int MOD = 1e9 + 7;
public:
    int countTexts(string pressedKeys) {
        int n = pressedKeys.size();
        // a => press once on a key.
        // b => press twice on the same key.
        // c => press thrice on the same key.
        // d => press four times on the same key.
        LL a = 1, b = 0, c = 0, d = 0;
        for (int i = 1; i < n; ++i) {
            char x = pressedKeys[i];
            if (x != pressedKeys[i - 1]) {
                a = (a + b + c + d) % MOD;
                b = 0, c = 0, d = 0;
            } else {
                LL t = (a + b + c + d) % MOD;
                if (x == '7' || x == '9') d = c;
                c = b;
                b = a;
                a = t;
            }
        }
        return (a + b + c + d) % MOD;
    }
};