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class Solution {
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  public List<List<Integer>> largeGroupPositions(String s) {
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    List<List<Integer>> res = new ArrayList<>();
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    List<Integer> tmp = new ArrayList<>();
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    int count = 1;
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    for (int i = 0; i < s.length() - 1; i++) {
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      // Increment the count until the next element is the same as the previous element. Ex: "aaa"
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      if (s.charAt(i) == s.charAt(i + 1)) {
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        count++;
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      }
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      // Add the first and last indices of the substring to the list when the next element is
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      // different from the previous element. Ex: "aaab"
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      else if (s.charAt(i) != s.charAt(i + 1) && count >= 3) {
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        // gives the starting index of substring
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        tmp.add(i - count + 1);
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        // gives the last index of substring
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        tmp.add(i);
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        res.add(tmp);
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        count = 1;
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        tmp = new ArrayList<>();
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      } else {
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        count = 1;
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      }
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    }
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    // Check for a large group at the end of the string. Ex: "abbb".
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    if (count >= 3) {
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      tmp.add(s.length() - count);
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      tmp.add(s.length() - 1);
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      res.add(tmp);
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    }
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    return res;
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  }
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}