Magical String LeetCode solution in C++

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class Solution {
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 public:
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  int magicalString(int n) {
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    string s = "122";
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    int sz = 3;
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    int start = 2, lastDig = 2;
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    while (sz < n) {
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      int cnt = s[start++] - '0';
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      int currDig = (lastDig == 2) ? 1 : 2;
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      for (int i = 0; i < cnt; i++) {
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        s.push_back('0' + currDig);
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        sz++;
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      }
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      lastDig = currDig;
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    }
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    int ans = 0;
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    for (int i = 0; i < n; i++) {
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      if (s[i] == '1') {
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        ans++;
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      }
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    }
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    return ans;
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  }
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};