fixed multiset find

In last day’s log I refer to multiset delete as

multiset.erase(multiset.find());

which will result in two finding operation in one call, making it rather inefficient. To avoid this, just using

multiset.erase(index);

This will return an iterator to a single element once called.To get all the location of a certain index, consider using

multiset.equal_range();

instead.

See more at: http://www.cplusplus.com/reference/set/multiset/find/