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equal_range(应用于有序区间)

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描写叙述：利用二分查找找到一个区间，区间里的全部值都等于给定值，返回的是一个pair。
分别存储区间的上界迭代器和下界迭代器

源代码：

template <class ForwardIterator, class T>
inline pair<ForwardIterator, ForwardIterator>
equal_range(ForwardIterator first, ForwardIterator last, const T& value) {return __equal_range(first, last, value, distance_type(first),iterator_category(first));
}// ForwardIterator 版本号
template <class ForwardIterator, class T, class Distance>
pair<ForwardIterator, ForwardIterator>
__equal_range(ForwardIterator first, ForwardIterator last, const T& value,Distance*, forward_iterator_tag) {Distance len = 0;distance(first, last, len);Distance half;ForwardIterator middle, left, right;while (len > 0) { // 奇怪？ 为什么不直接用 lower_bound 、 upper_bound ， 而是等找到值再用？// --> 我认为是效率方面的考虑。

先找 value ，这时左右两个区间可能已经缩小了很多。 // 再利用 lower_bound 和 upper_bound 代价小非常多 half = len >> 1; middle = first; advance(middle, half); if (*middle < value) { first = middle; ++first; len = len - half - 1; } else if (value < *middle) len = half; else { left = lower_bound(first, middle, value); advance(first, len); right = upper_bound(++middle, first, value); return pair<ForwardIterator, ForwardIterator>(left, right); } } return pair<ForwardIterator, ForwardIterator>(first, first); } // RandomAccessIterator 版本号 template <class RandomAccessIterator, class T, class Distance> pair<RandomAccessIterator, RandomAccessIterator> __equal_range(RandomAccessIterator first, RandomAccessIterator last, const T& value, Distance*, random_access_iterator_tag) { Distance len = last - first; Distance half; RandomAccessIterator middle, left, right; while (len > 0) { half = len >> 1; middle = first + half; if (*middle < value) { first = middle + 1; len = len - half - 1; } else if (value < *middle) len = half; else { left = lower_bound(first, middle, value); right = upper_bound(++middle, first + len, value); return pair<RandomAccessIterator, RandomAccessIterator>(left, right); } } return pair<RandomAccessIterator, RandomAccessIterator>(first, first); }

演示样例：

int main()
{int A[] = { 1, 2, 3, 3, 3, 5, 8 };const int N = sizeof(A) / sizeof(int);for (int i = 2; i <= 4; ++i) {pair<int*, int*> result = equal_range(A, A + N, i);cout << endl;cout << "Searching for " << i << endl;cout << "  First position where " << i << " could be inserted: "<< result.first - A << endl;cout << "  Last position where " << i << " could be inserted: "<< result.second - A << endl;if (result.first < A + N)cout << "  *result.first = " << *result.first << endl;if (result.second < A + N)cout << "  *result.second = " << *result.second << endl;}
}  

/*
The output is:
Searching for 2First position where 2 could be inserted: 1Last position where 2 could be inserted: 2*result.first = 2*result.second = 3Searching for 3First position where 3 could be inserted: 2Last position where 3 could be inserted: 5*result.first = 3*result.second = 5Searching for 4First position where 4 could be inserted: 5Last position where 4 could be inserted: 5*result.first = 5*result.second = 5*/