Count Number of Texts LeetCode solution in C++

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class Solution {
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 public:
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  int mod = 1e9 + 7;
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  int solve(string &str, int idx) {
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    if (idx == str.length()) return 1;
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    int maxKeyPress = (str[idx] == '7' || str[idx] == '9') ? 4 : 3;
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    long long currIndex = idx, pressFrequency = 1, ans = 0;
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    while (pressFrequency <= maxKeyPress && str[currIndex] == str[idx]) {
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      ++currIndex;
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      ++pressFrequency;
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      ans += solve(str, currIndex) % mod;
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    }
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    return ans % mod;
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  }
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  int countTexts(string pressedKeys) {
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    return solve(pressedKeys, 0) % mod;
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  }
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};