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class Solution {
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 public:
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  void add1(vector<int> &arr) {
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    if (arr.back() < 9) {
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      arr.back()++;
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      return;
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    }
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    reverse(begin(arr), end(arr));
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    int carry = 1;
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    for (int i = 0; i < arr.size(); i++) {
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      if (arr[i] < 9) {
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        arr[i]++;
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        carry = 0;
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        break;
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      }
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      arr[i] = 0;
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      carry = 1;
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    }
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    if (carry == 1) arr.push_back(1);
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    reverse(begin(arr), end(arr));
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  }
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  int compress(vector<char> &chars) {
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    int i = 0;
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    for (int j = 0; j < chars.size(); j++) {
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      if (j == chars.size() - 1 or chars[j] != chars[j + 1]) {
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        chars[i++] = chars[j];
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      }
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      else {
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        vector<int> cnt{0};
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        char ch = chars[j];
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        while (j < chars.size() and chars[j] == ch) {
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          j++;
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          add1(cnt);
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        }
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        j--; // bcoz j will be incremented in for loop updation condition.
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        chars[i++] = ch;
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        for (auto &it : cnt) chars[i++] = '0' + it;
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      }
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    }
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    chars.erase(chars.begin() + i, chars.end());
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    return i;
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  }