C++ STL中set与unordered_set对比

                |     set             | unordered_set
---------------------------------------------------------
Ordering        | increasing  order   | no ordering
                | (by default)        |

Implementation  | Self balancing BST  | Hash Table
                | like Red-Black Tree |  

search time     | log(n)              | O(1) -> Average 
                |                     | O(n) -> Worst Case

Insertion time  | log(n) + Rebalance  | Same as search

Deletion time   | log(n) + Rebalance  | Same as search

• 我们需要有序的数据。
• 我们必须(按顺序)打印/访问这些数据。
• 我们需要元素的前身/继承者。
• 因为set是有序的，所以可以对set元素使用binary_search()、lower_bound()和upper_bound()等函数。这些函数不能用于unordered_set()。

使用unordered_set

• 我们需要保留一组不同的元素，不需要排序。
• 我们需要单元素访问i.e。没有遍历。

例子:

set:
Input  :  1, 8, 2, 5, 3, 9
Output :  1, 2, 3, 5, 8, 9

Unordered_set:
Input  : 1, 8, 2, 5, 3, 9
Output : 9 3 1 8 2 5 

如果你想查看c++ STL中set和unordered_set的实现细节，请参阅set Vs Map。Set允许按有序顺序遍历元素，而Unordered_set不允许按有序顺序遍历元素。

// Program to print elements of set
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    set<int> s;
    s.insert(5);
    s.insert(1);
    s.insert(6);
    s.insert(3);
    s.insert(7);
    s.insert(2);
 
    cout << "Elements of set in sorted order: \n";
    for (auto it : s)
        cout << it << " ";
 
    return 0;
}

输出:

Elements of set in sorted order: 
1 2 3 5 6 7

// Program to print elements of set
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    unordered_set<int> s;
    s.insert(5);
    s.insert(1);
    s.insert(6);
    s.insert(3);
    s.insert(7);
    s.insert(2);
 
    cout << "Elements of unordered_set: \n";
    for (auto it : s)
        cout << it << " ";
 
    return 0;
}

输出:

Elements of unordered_set: 
2 7 5 1 6 3

前任/继任者 : Set可以修改为查找前任或继任者，而Unordered_set不允许查找前任/继任者。

// Program to print inorder predecessor and inorder successor
#include <bits/stdc++.h>
using namespace std;
 
set<int> s;
 
void inorderPredecessor(int key)
{
    if (s.find(key) == s.end()) {
        cout << "Key doesn't exist\n";
        return;
    }
 
    set<int>::iterator it;
    it = s.find(key); // get iterator of key
 
    // If iterator is at first position
    // Then, it doesn't have predecessor
    if (it == s.begin()) {
        cout << "No predecessor\n";
        return;
    }
 
    --it; // get previous element
    cout << "predecessor of " << key << " is=";
    cout << *(it) << "\n";
}
 
void inorderSuccessor(int key)
{
    if (s.find(key) == s.end()) {
        cout << "Key doesn't exist\n";
        return;
    }
 
    set<int>::iterator it;
    it = s.find(key); // get iterator of key
    ++it; // get next element
 
    // Iterator points to NULL (Element does
    // not exist)
    if (it == s.end())
    {
        cout << "No successor\n";
        return;
    }
    cout << "successor of " << key << " is=";
    cout << *(it) << "\n";
}
 
int main()
{
    s.insert(1);
    s.insert(5);
    s.insert(2);
    s.insert(9);
    s.insert(8);
 
    inorderPredecessor(5);
    inorderPredecessor(1);
    inorderPredecessor(8);
    inorderSuccessor(5);
    inorderSuccessor(2);
    inorderSuccessor(9);
 
    return 0;
}

输出:

predecessor of 5 is=2
No predecessor
predecessor of 8 is=5
successor of 5 is=8
successor of 2 is=5
No successor

让我们用表格的形式来看看它们的区别: