How to solve without two loop?

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int maxTrailing(vector s) {

int mn=s[0];
int mx=-19;
for(int i=1;i<s.size();++i)
{
    mx=max(mx,s[i]-mn);
    mn=min(mn,s[i]);
}
if(mx<=0)
{
    mx=-1;
}
return mx;

}