Summarize consecutive character runs

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Problem

Implement summarize_runs(text), which returns (character, count) for each maximal consecutive run. For example, "aaabbcaa" becomes [("a", 3), ("b", 2), ("c", 1), ("a", 2)]. Return an empty list for an empty string.

Reveal answer or reference solution

Keep the current character and count. When the next character differs, emit the completed run and reset the state. Emit the final run after the loop. The algorithm takes $O(n)$ time, $O(1)$ auxiliary space, and $O(r)$ output space for $r$ runs.

def summarize_runs(text: str) -> list[tuple[str, int]]:
    if text == "":
        return []
    runs: list[tuple[str, int]] = []
    current = text[0]
    count = 1
    for index in range(1, len(text)):
        character = text[index]
        if character == current:
            count += 1
        else:
            runs.append((current, count))
            current = character
            count = 1
    runs.append((current, count))
    return runs

Public tests

summarize_runs("") → []
summarize_runs("z") → [('z', 1)]
summarize_runs("aaabbcaa") → [('a', 3), ('b', 2), ('c', 1), ('a', 2)]
summarize_runs("ab") → [('a', 1), ('b', 1)]

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Summarize consecutive character runs

Problem

Public tests

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