C++ stl算法——partition

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函数声明

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template< class ForwardIt, class UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );

作用

对[first, last)元素进行处理，使得满足p的元素移到[first, last)前部，不满足的移到后部，返回第一个不满足p元素所在的迭代器，如果都满足的话返回last

可能的实现代码

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template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
    first = std::find_if_not(first, last, p);
    if (first == last) return first;
 
    for(ForwardIt i = std::next(first); i != last; ++i){
        if(p(*i)){
            std::iter_swap(i, first);
            ++first;
        }
    }
    return first;
}

复杂度

  1. 一次循环，所以时间复杂度为O(N)
  2. 最坏会进行O(N)次swap, (有且仅有第一个为不满足的情况)

示例

使用partition进行一般操作，并用partition实现了一个快排

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#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
#include <forward_list>
 
template <class ForwardIt>
 void quicksort(ForwardIt first, ForwardIt last)
 {
    if(first == last) return;
    auto pivot = *std::next(first, std::distance(first,last)/2);
    ForwardIt middle1 = std::partition(first, last, 
                         [pivot](const auto& em){ return em < pivot; });
    quicksort(first, middle1);
    quicksort(std::next(middle1, 1), last);
 }
 
int main()
{
    std::vector<int> v = {0,1,2,3,4,5,6,7,8,9};
    std::cout << "Original vector:\n    ";
    for (int elem : v) std::cout << elem << ' ';
 
    auto it = std::partition(v.begin(), v.end(), [](int i){return i % 2 == 0;});
 
    std::cout << "\nPartitioned vector:\n    ";
    std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
    std::cout << " * ";
    std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
 
    std::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list:\n    ";
    for(int n : fl) std::cout << n << ' ';
    std::cout << '\n';  
 
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "Sorted using quicksort:\n    ";
    for(int fi : fl) std::cout << fi << ' ';
    std::cout << '\n';
}

输出

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Original vector:
    0 1 2 3 4 5 6 7 8 9 
Partitioned vector:
    0 8 2 6 4  *  5 3 7 1 9 
Unsorted list:
    1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92 
Sorted using quicksort:
    -8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92