count
计算first和last之间与value相等于元素个数
template <class InputIterator,class EqualityComparable> typename iterator_traits<_InputIterator>::difference_type count (InputIterator first,InputIterator last,const EqualityComparable &value);
count_if
与count相似，但是count_if是计算满足某种条件的元素的个数，满足pred(*i)为true的iterator i的个数
template<typename _InputIterator, typename _Predicate>
typename iterator_traits<_InputIterator>::difference_type
__count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred)
{
typename iterator_traits<_InputIterator>::difference_type __n = ;
for (; __first != __last; ++__first)
if (__pred(__first))
++__n;
return __n;
}