Smallest leter greater than target

Accepted

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class Solution {
public:
    char nextGreatestLetter(vector<char>& letters, char target) {
      // if the letter is bigger than the biggest in our sorted input then
      // just pick the smallest one since letters "wrap around"
      if (letters.back() <= target) return letters[0];

      int low = 0;
      int high = letters.size() - 1;
      char res;

      while (low <= high) {
        int mid = low + (high - low)/2;
        if (letters[mid] <= target) {
          low = mid + 1;
        } else {
          res = letters[mid];
          high = mid - 1;
        }
      }

      return res;
    }
};