compressString function

String compressString(String input)

Compresses the input string by replacing consecutive repeated characters with the character followed by the count of repetitions.

If the compressed string is not shorter than the original, returns the original.

This implementation runs in O(n) time and O(n) space, where n is the length of the input string.

Example:

String compressed = compressString("aaabbc");
print(compressed); // Outputs: "a3b2c1"

Implementation

String compressString(String input) {
  if (input.isEmpty) return input;

  final buffer = StringBuffer();
  int count = 1;

  // Iterate from second character to the end
  for (int i = 1; i < input.length; i++) {
    if (input[i] == input[i - 1]) {
      count++;
    } else {
      buffer
        ..write(input[i - 1])
        ..write(count);
      count = 1;
    }
  }

  // Append the last character and its count
  buffer
    ..write(input[input.length - 1])
    ..write(count);

  final compressed = buffer.toString();

  // Return original if compression doesn't reduce size
  return compressed.length < input.length ? compressed : input;
}