algorithm 03 partiton is_partitioned

template <class BidirectionalIterator, class UnaryPredicate>
  BidirectionalIterator partition (BidirectionalIterator first,
                                   BidirectionalIterator last, UnaryPredicate pred);
功能：重新排列，使pred(*first)返回true的放在前面，返回false放在后面。函数返回是false的第一个元素
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is_partitioned

如果使pred(*first)都返回true的放在前面，返回false都放在后面，函数返回true，否则返回false