text behaves like a match-three game: whenever exactly run equal letters stand side by side, that block vanishes and the left and right parts of the string join up. Joining may create new blocks, and they vanish too.
Keep going until nothing more can be removed and return what is left. The result does not depend on the order of removals.text="xyyyxxz", run=3 → 'z'
"yyy" collapses into "xxxz", then "xxx" collapses and "z" is left.
text="aabba", run=2 → 'a'
Remove "aa" and "bb", leaving "a".
text="abab", run=2 → 'abab'
No two equal letters stand together, so the string stays the same.
text consists of lowercase English letterstext="xyyyxxz", run=3
'z'
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