文字列のith以降で文字cが出現する最小のindex

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auto res = nextCharIndex(s);

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vector<vector<int>> nextCharIndex(string &_s) {
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  vector m(_s.size() + 1, vector<int>(26));
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  for (int i = 0; i < 26; i++)
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    m[_s.size()][i] = _s.size();
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  for (int i = _s.size() - 1; i >= 0; i--)
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    for (int j = 0; j < 26; j++)
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      if (_s[i] - 'a' == j)
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        m[i][j] = i;
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      else
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        m[i][j] = m[i + 1][j];
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  return m;
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}