C++ find_by_order()用法及代碼示例

這個find_by_order()是一個內置的函數有序集這是一個基於策略的數據結構在C++中。基於策略的數據結構不屬於C++標準模板庫但的g ++編譯器支持它們。

有序集是g ++中基於策略的數據結構，可以按排序的順序維護唯一的元素。它以O(logN)複雜度執行STL中Set所執行的所有操作。除此之外，還以O(logN)複雜度執行以下兩個操作：

order_of_key (K): Number of items strictly smaller than K.
find_by_order(k): K^th element in a Set (counting from zero).

find_by_order()函數接受一個稱為K的鍵作為參數，並將迭代器返回給K^th集合中最大的元素。

例子：

Considering a Set S = {1, 5, 6, 17, 88},
s.find_by_order(0):Returns the 0^th largest element, i.e. the minimum element, i.e. 1.
s.find_by_order(2):Returns the 2^nd largest element, i.e. 6.

Note: If K >= N, where N is the size of the set, then the function returns either 0 or in some compilers, the iterator to the smallest element.

下麵是C++中find_by_order()函數的實現：

C++ 14


// C++ program to implement find_by_order()
// for Policy Based Data Structures
  
#include <bits/stdc++.h>
  
// Importing header files
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
  
// Declaring Ordered Set
typedef tree<int, null_type, less<int>, rb_tree_tag,
        tree_order_statistics_node_update>
    pbds;
  
// Driver Code
int main()
{
    
      int arr[] = {1, 5, 6, 17, 88};
     int n = sizeof(arr)/sizeof(arr[0]);
    
    pbds S;
    
      // Traverse the array
    for (int i = 0; i < n; i++) {
        
          // Insert array elements
          // into the ordered set
        S.insert(arr[i]);
    }
    
      // Returns iterator to 0-th
      // largest element in the set
    cout << *S.find_by_order(0) << " ";
    
    // Returns iterator to 2-nd
      // largest element in the set
    cout << *S.find_by_order(2);
  
    return 0;
}

輸出：

1 6

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