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equal_range(应用于有序区间)
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描述:利用二分查找找到一个区间,区间里的所有值都等于给定值,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 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? // --> 我觉得是效率方面的考虑。先找 value ,这时左右两个区间可能已经缩小了许多, // 再利用 lower_bound 和 upper_bound 代价小很多 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 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); }
示例:
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 2 First position where 2 could be inserted: 1 Last position where 2 could be inserted: 2 *result.first = 2 *result.second = 3 Searching for 3 First position where 3 could be inserted: 2 Last position where 3 could be inserted: 5 *result.first = 3 *result.second = 5 Searching for 4 First position where 4 could be inserted: 5 Last position where 4 could be inserted: 5 *result.first = 5 *result.second = 5*/
STL 源码剖析 算法 stl_algo.h -- equal_range,布布扣,bubuko.com
时间: 2024-10-14 08:22:58