STL 源碼剖析 演算法 stl_algo.h -- equal_range

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equal_range(應用於有序區間)

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描述：利用二分尋找找到一個區間，區間裡的所有值都等於給定值，返回的是一個pair，
分別儲存區間的上界迭代器和下界迭代器

源碼：

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？ 為什麼不直接用 lower_bound 、 upper_bound ， 而是等找到值再用？// --> 我覺得是效率方面的考慮。先找 value ，這時左右兩個區間可能已經縮小了許多，// 再利用 lower_bound 和 upper_bound 代價小很多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);}

樣本：

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