Run-Length String Compression

Compress contiguous runs in a string from left to right: emit a character alone when its run length is one, otherwise emit the character followed by its count. Requirements Scan the input string from left to right and group contiguous identical characters. If a character appears once in its run, append only that character. If a character appears more than once consecutively, append the character followed by the run length. Return the concatenation of all encoded runs. Examples The interview input was aaaaabbbccca; produce its compressed form under the rules above. Notes Maintain the start of the current run and advance an end pointer until the character changes. Emit the character and append the run length only when it exceeds one. Flush the final run after the scan; omitting that flush is the most common boundary bug. Applying the stated rules to the interview input yields a5b3c3a. Time complexity is O(n); the result uses O(n) space in the worst case. Preparation Implement the two-pointer scan once with an explicit final-run flush and once with a sentinel-style loop. Trace the interview input by hand and verify the four runs and the result a5b3c3a. Test an empty string, one character, alternating characters, and a run whose count has multiple digits; clarify the empty-input contract if it is not stated.