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Size: a a a
AP
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arr.reduce(function(acc, el) {
acc[el] = true;
return acc;
},[]).slice(1).findIndex(el => !el) + 1;
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V
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src.reduce((acc, el) => acc - el, (src.length + 1 + (src.length + 1) ** 2) / 2)
типа того, что ли?V