STL set 与 multiset 2016.10.25

参考：《21天学通C++》

一、简介

容器 set 和 multiset 让程序员能够快速查找键，键是存储在一维容器中的值

set 和 multiset 之间的区别在于，后者可存储重复的值，而前者只能存储唯一的值

为实现快速搜索，STL set 和 multiset 的内部结构像一棵二叉树，这意味着将元素插入到 set 或 multiset 时将对其进行排序，以提高查找速度

这还意味着不像 vector 那样可以使用其它元素替换给定位置的元素，位于 set 中特定位置的元素不能替换为值不同的新元素

这是因为 set 将把新元素同二叉树中的其它元素进行比较，进而将其放在其它位置

二、基本操作

1、插入

#include <iostream>
#include <set>

using namespace std;

template <typename Container> void PrintContents(const Container & stlContainer);

set<int> Set;
multiset<int> Mset;

int main()
{
    Set.insert(60);
    Set.insert(-1);
    Set.insert(3000);
    cout << "Writing the contents of the set to the screen" << endl;
    PrintContents(Set);
    Mset.insert(Set.begin(), Set.end());
    Mset.insert(3000);
    cout << "Writing the contents of the multiset to the screen" << endl;
    PrintContents(Mset);
    cout << "Number of instances of the multiset are : '";
    cout << Mset.count(3000) << "'" << endl;
    return 0;
}

template <typename Container> void PrintContents(const Container & stlContainer)
{
    typename Container::const_iterator itr = stlContainer.begin();
    while (itr != stlContainer.end()) {
        cout << *itr << endl;
        ++itr;
    }
    cout << endl;
}

输出：

Writing the contents of the set to the screen
-1
60
3000

Writing the contents of the multiset to the scre
-1
60
3000
3000

Number of instances of the multiset are : '2'

2、查找

#include <iostream>
#include <set>

using namespace std;

set<int> Set;

int main()
{
    Set.insert(43);
    Set.insert(78);
    Set.insert(-1);
    Set.insert(124);
    set<int>::const_iterator itr;
    for (itr = Set.begin(); itr != Set.end(); ++itr) {
        cout << *itr << endl;
    }
    set<int>::iterator itr_found = Set.find(-1);
    if (itr_found != Set.end()) {
        cout << "Element " << *itr_found << " found!" << endl;
    } else {
        cout << "Element not found in set!" << endl;
    }
    set<int>::iterator itr_nofound = Set.find(12345);
    if (itr_nofound != Set.end()) {
        cout << "Element " << *itr_nofound << " found!" << endl;
    } else {
        cout << "Element 12345 not found in set!" << endl;
    }
    return 0;
}

输出：

-1
43
78
124
Element -1 found!
Element 12345 not found in set!

3、删除

#include <iostream>
#include <set>

using namespace std;

multiset<int> Mset;

int main()
{
    Mset.insert(43);
    Mset.insert(78);
    Mset.insert(78);
    Mset.insert(-1);
    Mset.insert(124);
    multiset<int>::const_iterator itr;
    cout << "multiset contains " << Mset.size() << " elements.";
    cout << "These are: " << endl;
    for (itr = Mset.begin(); itr != Mset.end(); ++itr) {
        cout << *itr << endl;
    }
    cout << "Please enter a number to be erased from the set" << endl;
    int num_erase = 0;
    cin >> num_erase;
    cout << "Erasing " << Mset.count(num_erase) << " instances of value " << num_erase << endl;
    Mset.erase(num_erase);
    cout << "multiset contains " << Mset.size() << " elements.";
    cout << "These are: " << endl;
    for (itr = Mset.begin(); itr != Mset.end(); ++itr) {
        cout << *itr << endl;
    }
    return 0;
}

输出：

multiset contains 5 elements.These are:
-1
43
78
78
124
Please enter a number to be erased from the set
78
Erasing 2 instances of value 78
multiset contains 3 elements.These are:
-1
43
124

注意：

函数 erase() 被重载了，可对迭代器（如 find 返回的迭代器）调用 erase() 以删除找到的元素，如下所示

    multiset<int>::iterator itr = Mset.find(num_erase);
    if (itr != Mset.end()) {
        Mset.erase(itr);
    } else {
        cout << "Element not found!" << endl;
    }

同样，erase() 还可用于从 multiset 中删除指定范围内的元素：

    multiset<int>::iterator itr = Mset.find(value);
    if (itr != Mset.end()) {
        Mset.erase(Mset.begin(), itr);
    }

上述代码删除从头到值为 value 的所有元素（不包含 value）

要清空 set 和 multiset 的内容，可使用其成员函数 clear()

三、问与答

1、问：如何声明一个其元素按降序排列的整型 set ？

答：set<int> 定义一个整型 set ，这种 set 使用默认排序谓词 std::less<T> 将元素按升序排列

也可将其定义为 set<int, less<int>>

要按降序排列，应将 set 定义为 set<int, greater<int>>

HDU 5700 区间交

参考：http://blog.youkuaiyun.com/tc_to_top/article/details/51476593

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <vector>
#include <stack>
#include <map>
#include <cmath>
#include <cctype>
#include <bitset>
#include <ctime>
#include <set>

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;
typedef pair<int, int> Pair;

const ull mod = 1e9 + 7;
const int INF = 0x7fffffff;
const int maxn = 1e5 + 10;

ll a[maxn], sum[maxn], n, m, k, l, r, maxl = 0;
vector<ll> V[maxn];
multiset<ll> Mset;

int main()
{
#ifdef __AiR_H
    freopen("in.txt", "r", stdin);
#endif // __AiR_H
    while (scanf("%I64d %I64d %I64d", &n, &k, &m) != EOF) {
        for (ll i = 0; i < n; ++i) {
            V[i].clear();
        }
        Mset.clear();
        for (ll i = 1; i <= n; ++i) {
            scanf("%I64d", &a[i]);
            sum[i] = sum[i-1] + a[i];
        }
        while (m--) {
            scanf("%I64d %I64d", &l, &r);
            maxl = max(maxl, l);
            V[l].push_back(r);
        }
        ll ans = 0;
        for (ll i = 1; i <= maxl; ++i) {
            ll Size = V[i].size();
            for (ll j = 0; j < Size; ++j) {
                Mset.insert(V[i][j]);
            }
            while (Mset.size() > k) {
                Mset.erase(Mset.begin());
            }
            if (Mset.size() == k && *Mset.begin() >= i) {
                ans = max(ans, sum[*Mset.begin()] - sum[i-1]);
            }
        }
        printf("%I64d\n", ans);
    }
    return 0;
}

Codeforces 722C Destroying Array

参考：http://www.cnblogs.com/flipped/p/5928466.html

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <vector>
#include <stack>
#include <map>
#include <set>
#include <cmath>
#include <cctype>
#include <bitset>
#include <ctime>

using namespace std;

#define lson low, mid, _id<<1
#define rson mid+1, high, _id<<1|1

typedef long long ll;
typedef pair<ll, ll> Pair;

const ll maxn = 1e5 + 10;

set<Pair> Set;
multiset<ll> Mset;
ll n, p;
ll a[maxn], sum[maxn];

int main()
{
#ifdef __AiR_H
    freopen("in.txt", "r", stdin);
#endif // __AiR_H
    scanf("%I64d", &n);
    for (ll i = 1; i <= n; ++i) {
        scanf("%I64d", &a[i]);
        sum[i] = sum[i - 1] + a[i];
    }
    Set.insert(make_pair(n, 1));
    Mset.insert(sum[n]);
    for (ll i = 1; i <= n; ++i) {
        scanf("%I64d", &p);
        set<Pair>::iterator itr = Set.lower_bound(make_pair(p, 1));
        Mset.erase(Mset.find(sum[itr->first] - sum[itr->second - 1]));
        Set.insert(make_pair(p - 1, itr->second));
        Mset.insert(sum[p - 1] - sum[itr->second - 1]);
        Set.insert(make_pair(itr->first, p + 1));
        Mset.insert(sum[itr->first] - sum[p]);
        Set.erase(itr);
        multiset<ll>::reverse_iterator mitr = Mset.rbegin();
        ll ans = *mitr;
        printf("%I64d\n", ans);
    }
    return 0;
}