unordered_map/unordered_set的学习[unordered系列]

1.老生常谈_遍历

#pragma once
#define _CRT_SECURE_NO_WARNINGS 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

void test_unordered_set1()
{
	unordered_set<int> us;
	us.insert(1);
	us.insert(3);
	us.insert(2);
	us.insert(7);
	us.insert(2);

	unordered_set<int>::iterator it = us.begin();
	while (it != us.end())
	{
		cout << *it << " ";
		++it;
	}
	cout << endl;

	for (auto e : us)
	{
		cout << e << " ";
	}
	cout << endl;
}

void test_unordered_map()
{
	string s[] = { "陀螺", "陀螺", "洋娃娃", "陀螺", "洋娃娃", "洋娃娃", "陀螺",
				   "洋娃娃", "悠悠球", "洋娃娃", "悠悠球", "乐高" }; 
	unordered_map<string, int> um;
	for (auto& e : s)
	{
		um[e]++;
	}

	for (auto& e : um)
	{
		cout << e.first << ":" << e.second << endl;
	}
}


int main()
{
	
	const size_t N = 1000000;

	unordered_set<int> us;
	set<int> s;

	vector<int> v;
	v.reserve(N);
	srand(time(0));
	for (size_t i = 0; i < N; ++i)
	{
		//v.push_back(rand());   
		//v.push_back(rand()+i); 
		v.push_back(i);         
	}
	cout << "插入" << endl;
	//set插入
	size_t begin1 = clock();
	for (auto e : v)
	{
		s.insert(e);
	}
	size_t end1 = clock();
	cout << "          set insert:" << end1 - begin1 << endl;
	//unordered_set插入
	size_t begin2 = clock();
	for (auto e : v)
	{
		us.insert(e);
	}
	size_t end2 = clock();
	cout << "unordered_set insert:" << end2 - begin2 << endl;
	cout << endl;

	//存入数据
	cout << "存入数据量[初始值100w--去重后]" << endl;
	cout << "          set:" << s.size() << endl;
	cout << "unordered_set:" << us.size() << endl;
	cout << endl;

	cout << "查找" << endl;
	//set查找
	size_t begin3 = clock();
	for (auto e : v)
	{
		s.find(e);
	}
	size_t end3 = clock();
	cout << "          set find:" << end3 - begin3 << endl;
	//unordered_set查找
	size_t begin4 = clock();
	for (auto e : v)
	{
		us.find(e);
	}
	size_t end4 = clock();
	cout << "unordered_set find:" << end4 - begin4 << endl ;
	cout << endl;

	cout << "删除" << endl;
	//set删除
	size_t begin5 = clock();
	for (auto e : v)
	{
		s.erase(e);
	}
	size_t end5 = clock();
	cout << "          set erase:" << end5 - begin5 << endl;
	//unordered_set删除
	size_t begin6 = clock();
	for (auto e : v)
	{
		us.erase(e);
	}
	size_t end6 = clock();
	cout << "unordered_set erase:" << end6 - begin6 << endl << endl;
	cout << endl;

	return 0;
}

2.性能测试

3.OJ训练

3.1存在重复元素

存在重复元素

class Solution 
{
public:
	bool containsDuplicate(vector<int>& nums) 
	{
		unordered_set<int> us;
		for (auto x : nums) 
		{
			//找不到x 插入us
			if (us.find(x) == us.end())
				us.insert(x);
			//找到了--存在重复值
			else
				return true;
		}
		return false;
	}
};

3.2两个数组的交集Ⅱ

两个数组的交集Ⅱ

class Solution
{
public:
	vector<int> intersect(vector<int>& nums1, vector<int>& nums2)
	{
		//假定nums1数据个数少:不写也ok 
		// 作用在于到时用count遍历查找时次数少一些
		if (nums1.size() > nums2.size())
			return intersect(nums2, nums1);

		//插入到hashmap
		unordered_map <int, int> um;
		for (auto e : nums1)
		{
			//um中有e        e.second++
			//um中无e 插入e  e.second++
			//insert当插入相同key时--插入失败
			++um[e];
		}
		//遍历第二个数组 
		vector<int> v;
		for (auto e : nums2)
		{
			//若在数组1中也有--即是交集
			if (um.count(e))
			{
				//是交集--插入
				v.push_back(e);
				//下面三行的意义:
				//数组一中假设x出现2次 数组二x出现2次 
				// 每匹配一个 插入v中后 x次数--
				--um[e];
				if (um[e] == 0)
					um.erase(e);
			}
		}
		return v;
	}
};

3.3两句话中的不常见单词

两句话中的不常见单词

class Solution
{
public:
	vector<string> uncommonFromSentences(string s1, string s2)
	{
		unordered_map<string, int> um;

		//遍历字符串每一个字符 是字母保存 
		// 遇空格时 str定为一个单词 单词作为key插入 value值++
		//到最后value是1的即为obj

		string str = "";
		for (auto& e : s1)
		{
			if (e != ' ')
				str += e;
			else
			{
				um[str]++;
				str = "";
			}
		}
		um[str]++;
		str = "";

		for (auto& e : s2)
		{
			if (e == ' ')
			{
				um[str]++;
				str = "";
			}
			else
				str += e;
		}
		um[str]++;
		str = "";

		vector<string> v;
		for (auto& e : um)
		{
			if (e.second == 1)
				v.push_back(e.first);
		}
		return v;
	}
};

3.4两个数组的交集

两个数组的交集

class Solution 
{
public:
	vector<int> intersection(vector<int>& nums1, vector<int>& nums2) 
	{
		unordered_set<int> us1, us2;

		for (auto& e : nums1)
			us1.insert(e);

		for (auto& e : nums2)
			us2.insert(e);

		return get(us1, us2);
	}

	vector<int> get(unordered_set<int>& us1, unordered_set<int>& us2) 
	{
		if (us1.size() > us2.size())
			return get(us2, us1);

		vector<int> v;
		for (auto& e : us1)
		{
			if (us2.count(e))
				v.push_back(e);
		}
		return v;
	}
};

3.5在长度2N的数组中找出重复N次的元素

在长度2N的数组中找出重复N次的元素

class Solution
{
public:
	int repeatedNTimes(vector<int>& nums) 
	{
		unordered_set<int> us;
		for (auto& e : nums) 
		{
			if (us.count(e)) 
				return e;
			us.insert(e);
		}
		return -1;
	}
};