STL 源代码剖析 算法 stl_algo.h -- equal_range

Posted 2020-06-13

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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 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*/