Find Smallest Letter Greater Than Target LeetCode solution in C++

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class Solution {
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 public:
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  char nextGreatestLetter(vector<char> &letters, char target) {
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    int siz = letters.size();
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    bool isPresent = false;
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    char ans;
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    char temp = target;
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    if (target == letters[siz - 1]) return letters[0];
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    for (int i = 0; i < siz - 1; i++) {
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      if (target == letters[i] && target != letters[i + 1]) {
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        ans = letters[i + 1];
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        isPresent = true;
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        break;
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      }
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    }
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    // if target not in letters
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    while (!isPresent) {
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      temp = temp + 1;
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      isPresent = binary_search(letters.begin(), letters.end(), temp); // used STL !!
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      if (isPresent) {
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        ans = temp;
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        break;
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      }
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    }
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    return ans;
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  }
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};