lower_bound()与upper_bound()

所在头文件：#include<algorithm>

函数分类：Binary search (operating on partitioned/sorted ranges)

函数功能：lower_bound()返回一个迭代器指针，指向val出现在这个被查找序列中出现的第一个位置；upper_bound()返回一个迭代器指针，指向val出现在这个被查找序列中出现的最后一个位置的后一个位置。

lower_bound()的函数模板功能相当于：

template <class ForwardIterator, class T>
ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
  ForwardIterator it;
  iterator_traits<ForwardIterator>::difference_type count, step;
  count = distance(first,last);
  while (count>0)
  {
    it = first; step=count/2; advance (it,step);
    if (*it<val) 
    {                 // or: if (comp(*it,val)), for version (2)
      first=++it;
      count-=step+1;
    }
    else count=step;
  }
  return first;
}

upper_bound()的函数模板功能相当于：

<pre name="code" class="cpp">template <class ForwardIterator, class T>
ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
  ForwardIterator it;
  iterator_traits<ForwardIterator>::difference_type count, step;
  count = std::distance(first,last);
  while (count>0)
  {
    it = first; step=count/2; std::advance (it,step);
    if (!(val<*it))                 // or: if (!comp(val,*it)), for version (2)
    { 
      	first=++it; count-=step+1;  
    }
    else count=step;
  }
  return first;
}

函数模板：

#include <bits/stdc++.h>
using namespace std;


int lower(int *array, int b, int e, int val)
{
    int first = b;
    int count = e - b;
    while (count > 0)
    {
        int it = first;
        int step = count / 2;
        it = it + step;
        if (array[it] < val)
        {
            first = ++it;
            count = count - (step + 1);
        }
        else
        {
            count = step;
        }
    }
    return first;
}




int upper(int *array, int b, int e, int val)
{
    int first = b;
    int count = e - b;
    while (count > 0)
    {
        int it = first;
        int step = count / 2;
        it = it + step;
        if (array[it] <= val)
        {
            first = ++it;
            count = count - (step + 1);
        }
        else
        {
            count = step;
        }
    }
    return first;
}


int main()
{
    int array[] = {0,1,2,3,4,5,5,5,5,5,6,6,6,7,8,9,10,11,11,11,12,12,13,13};
    int e = sizeof(array) / sizeof(int);
    int ans1 = lower(array, 0, e - 1, 11);
    int ans2 = upper(array, 0, e - 1, 11);
    printf("%d\n", ans1);
    printf("%d\n", ans2);
    printf("-------------\n");
    int ans3 = lower(array, 0, e - 1, 13);
    int ans4 = upper(array, 0, e - 1, 13);
    printf("%d\n", ans3);
    printf("%d\n", ans4);
    printf("-------------\n");
    int ans5 = lower(array, 0, e - 1, 100);
    int ans6 = upper(array, 0, e - 1, 100);
    printf("%d\n", ans5);
    printf("%d\n", ans6);
    printf("-------------\n");


    return 0;
}