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1 Multimap 概述

Multimap 类似于map，但多个元素可以具有相同的键。此外，在这种情况下，键值和映射值对不必是唯一的。关于 multimap 需要注意的一件重要事情是 multimap 始终保持所有键的排序顺序。multimap 的这些特性使其在竞争性编程中非常有用。

与Multimap 相关的一些基本功能：

begin() – 将迭代器返回到Multimap 中的第一个元素
end() – 将迭代器返回到Multimap 中最后一个元素之后的理论元素
size() – 返回Multimap 中的元素数量
max_size() – 返回Multimap 可以容纳的最大元素数
empty() – 返回Multimap 是否为空
pair insert(keyvalue,multimapvalue) – 向Multimap 添加一个新元素

1 Multimap 代码示例

用于说明上述功能的 C++ 实现：

// CPP Program to demonstrate the implementation of multimap
#include 
#include 
#include 
using namespace std;
 
// Driver Code
int main()
{
    multimap<int, int> gquiz1; // empty multimap container
 
    // insert elements in random order
    gquiz1.insert(pair<int, int>(1, 40));
    gquiz1.insert(pair<int, int>(2, 30));
    gquiz1.insert(pair<int, int>(3, 60));
    gquiz1.insert(pair<int, int>(6, 50));
    gquiz1.insert(pair<int, int>(6, 10));
 
    // printing multimap gquiz1
    multimap<int, int>::iterator itr;
    cout << "\nThe multimap gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // adding elements randomly,
    // to check the sorted keys property
    gquiz1.insert(pair<int, int>(4, 50));
    gquiz1.insert(pair<int, int>(5, 10));
 
    // printing multimap gquiz1 again
 
    cout << "\nThe multimap gquiz1 after adding extra "
            "elements is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // assigning the elements from gquiz1 to gquiz2
    multimap<int, int> gquiz2(gquiz1.begin(), gquiz1.end());
 
    // print all elements of the multimap gquiz2
    cout << "\nThe multimap gquiz2 after assign from "
            "gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // remove all elements up to
    // key with value 3 in gquiz2
    cout << "\ngquiz2 after removal of elements less than "
            "key=3 : \n";
    cout << "\tKEY\tELEMENT\n";
    gquiz2.erase(gquiz2.begin(), gquiz2.find(3));
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
 
    // remove all elements with key = 4
    int num;
    num = gquiz2.erase(4);
    cout << "\ngquiz2.erase(4) : ";
    cout << num << " removed \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
 
    cout << endl;
 
    // lower bound and upper bound for multimap gquiz1 key =
    // 5
    cout << "gquiz1.lower_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.lower_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.lower_bound(5)->second
         << endl;
    cout << "gquiz1.upper_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.upper_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.upper_bound(5)->second
         << endl;
 
    return 0;
}
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输出
The multimap gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    6    50
    6    10


The multimap gquiz1 after adding extra elements is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    50
    5    10
    6    50
    6    10


The multimap gquiz2 after assign from gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    50
    5    10
    6    50
    6    10


gquiz2 after removal of elements less than key=3 : 
    KEY    ELEMENT
    3    60
    4    50
    5    10
    6    50
    6    10

gquiz2.erase(4) : 1 removed 
    KEY    ELEMENT
    3    60
    5    10
    6    50
    6    10

gquiz1.lower_bound(5) :     KEY = 5        ELEMENT = 10
gquiz1.upper_bound(5) :     KEY = 6        ELEMENT = 50
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Multimap的功能列表