C++ 算法库(5) 二分法查找操作

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二分法查找操作：binary_search判断范围中是否存在值等价于给定值的元素equal_range返回范围中值等于给定值的元素组成的子范围lower_bound返回指向范围中第一个值大于或等于给定值的元素的迭代器upper_bound返回指向范围中第一个值大于给定值的元素的迭代器

binary_search

判断范围中是否存在值等价于给定值的元素

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// binary_search example
#include <iostream>     // std::cout
#include <algorithm>    // std::binary_search, std::sort
#include <vector>       // std::vector

bool myfunction (int i,int j) { return (i<j); }

int main () {
  int myints[] = {1,2,3,4,5,4,3,2,1};
  std::vector<int> v(myints,myints+9);                         // 1 2 3 4 5 4 3 2 1

  // using default comparison:
  std::sort (v.begin(), v.end());

  std::cout << "looking for a 3... ";
  if (std::binary_search (v.begin(), v.end(), 3))
    std::cout << "found!\n"; else std::cout << "not found.\n";

  // using myfunction as comp:
  std::sort (v.begin(), v.end(), myfunction);

  std::cout << "looking for a 6... ";
  if (std::binary_search (v.begin(), v.end(), 6, myfunction))
    std::cout << "found!\n"; else std::cout << "not found.\n";

  return 0;
}

输出：

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looking for a 3... found!
looking for a 6... not found.

equal_range

返回范围中值等于给定值的元素组成的子范围

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// equal_range example
#include <iostream>     // std::cout
#include <algorithm>    // std::equal_range, std::sort
#include <vector>       // std::vector

bool mygreater (int i,int j) { return (i>j); }

int main () {
  int myints[] = {10,20,30,30,20,10,10,20};
  std::vector<int> v(myints,myints+8);                         // 10 20 30 30 20 10 10 20
  std::pair<std::vector<int>::iterator,std::vector<int>::iterator> bounds;

  // using default comparison:
  std::sort (v.begin(), v.end());                              // 10 10 10 20 20 20 30 30
  bounds=std::equal_range (v.begin(), v.end(), 20);            //          ^        ^

  // using "mygreater" as comp:
  std::sort (v.begin(), v.end(), mygreater);                   // 30 30 20 20 20 10 10 10
  bounds=std::equal_range (v.begin(), v.end(), 20, mygreater); //       ^        ^

  std::cout << "bounds at positions " << (bounds.first - v.begin());
  std::cout << " and " << (bounds.second - v.begin()) << '\n';

  return 0;
}

输出：

bounds at positions 2 and 5

lower_bound

返回指向范围中第一个值大于或等于给定值的元素的迭代器

upper_bound

返回指向范围中第一个值大于给定值的元素的迭代器

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// lower_bound/upper_bound example
#include <iostream>     // std::cout
#include <algorithm>    // std::lower_bound, std::upper_bound, std::sort
#include <vector>       // std::vector

int main () {
  int myints[] = {10,20,30,30,20,10,10,20};
  std::vector<int> v(myints,myints+8);           // 10 20 30 30 20 10 10 20

  std::sort (v.begin(), v.end());                // 10 10 10 20 20 20 30 30

  std::vector<int>::iterator low,up;
  low=std::lower_bound (v.begin(), v.end(), 20); //          ^
  up= std::upper_bound (v.begin(), v.end(), 20); //                   ^

  std::cout << "lower_bound at position " << (low- v.begin()) << '\n';
  std::cout << "upper_bound at position " << (up - v.begin()) << '\n';

  return 0;
}

输出：

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lower_bound at position 3
upper_bound at position 6

特别说明：函数的中文释义来自：http://classfoo.cn/cpp/head/76573_319/，例子来自：http://www.cplusplus.com/reference/algorithm/

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