STL 源码剖析 算法 stl_algo.h -- upper_bound

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upper_bound(应用于有序区间)

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描述：受STL区间前闭后开习惯的影响，upper_bound成功找到某个值时，

返回一个迭代器指向每一个"不大于 value "的元素的下一个位置，而不是指向 value 的迭代器，

或找不到，返回value 应该存在的位置

思路：

1.循环直到区间长度为 0 

2.如果 value < *middle，在前半段继续查找

3.如果 value >= *middle，在后半段继续查找 (等于的时候也会继续在后半段查找，所以能保证找到的是 upper bound)

源码：
template <class ForwardIterator, class T>
inline ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last,
const T& value) {
return __upper_bound(first, last, value, distance_type(first),
iterator_category(first));
}

// forward_iterator_tag 版本
template <class ForwardIterator, class T, class Distance>
ForwardIterator __upper_bound(ForwardIterator first, ForwardIterator last,
const T& value, Distance*,
forward_iterator_tag) {
Distance len = 0;
distance(first, last, len);
Distance half;
ForwardIterator middle;

while (len > 0) {
half = len >> 1;
middle = first;
advance(middle, half);
if (value < *middle)
len = half;
else {
first = middle;
++first;
len = len - half - 1;
}
}
return first;
}

// random_access_iterator_tag 版本
template <class RandomAccessIterator, class T, class Distance>
RandomAccessIterator __upper_bound(RandomAccessIterator first,
RandomAccessIterator last, const T& value,
Distance*, random_access_iterator_tag) {
Distance len = last - first;
Distance half;
RandomAccessIterator middle;

while (len > 0) {
half = len >> 1;
middle = first + half;
if (value < *middle)
len = half;
else { //因为大于等都是在后半段区间查找，所以最后找到的一定是 upper bound，而且存在 value 的话， first 最后指示的就是 value 的下一个位置
first = middle + 1;
len = len - half - 1;
}
}
return first;
}

示例：

int main()
{
int A[] = { 1, 2, 3, 3, 3, 5, 8 };
const int N = sizeof(A) / sizeof(int);

for (int i = 1; i <= 10; ++i) {
int* p = upper_bound(A, A + N, i);
cout << "Searching for " << i << ".  ";
cout << "Result: index = " << p - A << ", ";
if (p != A + N)
cout << "A[" << p - A << "] == " << *p << endl;
else
cout << "which is off-the-end." << endl;
}
}
/*
The output is:
Searching for 1.  Result: index = 1, A[1] == 2
Searching for 2.  Result: index = 2, A[2] == 3
Searching for 3.  Result: index = 5, A[5] == 5
Searching for 4.  Result: index = 5, A[5] == 5
Searching for 5.  Result: index = 6, A[6] == 8
Searching for 6.  Result: index = 6, A[6] == 8
Searching for 7.  Result: index = 6, A[6] == 8
Searching for 8.  Result: index = 7, which is off-the-end.
Searching for 9.  Result: index = 7, which is off-the-end.
Searching for 10.  Result: index = 7, which is off-the-end.
*/