<?php function isQuadtree($bits, &$i = 0){ if ($i >= str_length($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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