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481. Magical String 👎

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  int magicalString(int n) {
    string s = " 122";

    for (int i = 3; i <= n; ++i)
      if (i & 1)
        s.append(s[i] - '0', '1');
      else
        s.append(s[i] - '0', '2');

    return count(begin(s), begin(s) + n + 1, '1');
  }
};
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