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<?php function isQuadtree($bits, &$i = -1){ $i++; if ($i > strlen($bits)) return false; if ($bits[$i] === "1") { isQuadtree($bits, $i); isQuadtree($bits, $i); isQuadtree($bits, $i); return isQuadtree($bits, $i); } return true; } var_dump(isQuadtree("110000"));

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