What is the worst$-$case running time of this algorithm when the input vector nums has size n$?$

bool containsDuplicate$($const std::vector& nums$)\{$

std::set seen;

for $($int x : nums$)\{$

if $($seen$.$contains$($x$))\{$

return true;

$\}$

seen.insert$($x$)$;

$\}$

return false;

$\}$