一.unordered系列关联式容器
unordered_set 和 unordered_map 中序遍历都是无序的
二.unordered_set的介绍
cplusplius文档链接如下:
https://legacy.cplusplus.com/reference/unordered_set/unordered_set/
平时使用不用管
1.接口介绍
只能说,set和unordered_set的使用都是差不多的(90%),但是由于底层的差别(set(红黑树),unordered_set(哈希表)),有一些差别,如下:
从unordered_set没提供的rbegin()和rend()可以知道
2.代码测试
1.接口测试
cpp
#include<unordered_set>
#include<unordered_map>
#include<set>
#include<iostream>
#include <time.h>
#include <vector>
using namespace std;
void test_set1()
{
unordered_set<int> s = { 3,1,6,7,8,2 };
unordered_set<int>::iterator it = s.begin();
while (it != s.end())
{
cout << *it << " ";
++it;
}
cout << endl;
for (auto e : s)
{
cout << e << " ";
}
cout << endl;
}
这里验证了,我们的unordered_set插入数据是无序的
2.性能测试
cpp
int test_set2()
{
const size_t N = 10000000;
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()); // N比较大时，重复值比较多
//v.push_back(rand()+i); // 重复值相对少
v.push_back(i); // 没有重复，有序
}
size_t begin1 = clock();
for (auto e : v)
{
s.insert(e);
}
size_t end1 = clock();
cout << "set insert:" << end1 - begin1 << endl;
size_t begin2 = clock();
for (auto e : v)
{
us.insert(e);
}
size_t end2 = clock();
cout << "unordered_set insert:" << end2 - begin2 << endl;
int m1 = 0;
size_t begin3 = clock();
for (auto e : v)
{
auto ret = s.find(e);
if (ret != s.end())
{
++m1;
}
}
size_t end3 = clock();
cout << "set find:" << end3 - begin3 << "->" << m1 << endl;
int m2 = 0;
size_t begin4 = clock();
for (auto e : v)
{
auto ret = us.find(e);
if (ret != us.end())
{
++m2;
}
}
size_t end4 = clock();
cout << "unorered_set find:" << end4 - begin4 << "->" << m2 << endl;
cout << "插入数据个数：" << s.size() << endl;
cout << "插入数据个数：" << us.size() << endl << endl;
size_t begin5 = clock();
for (auto e : v)
{
s.erase(e);
}
size_t end5 = clock();
cout << "set erase:" << end5 - begin5 << endl;
size_t begin6 = clock();
for (auto e : v)
{
us.erase(e);
}
size_t end6 = clock();
cout << "unordered_set erase:" << end6 - begin6 << endl << endl;
return 0;
}
unordered_set有序性能低,set有序性能高
三.unordered_map的介绍
1.接口介绍
2.代码测试
cpp
void test_map1()
{
unordered_map<string, string> dict;
dict.insert({ "left", "左边" });
dict["sort"];
dict["sort"] = "排序";
}和map的基础功能是一样的
四.题目
1.
https://leetcode.cn/problems/n-repeated-element-in-size-2n-array/description/
cpp
class Solution {
public:
int repeatedNTimes(vector<int>& A) {
size_t N = A.size() / 2;
// 用unordered_map统计每个元素出现的次数
unordered_map<int, int> m;
for(auto e : A)
m[e]++;
// 找出出现次数为N的元素
for(auto& e : m)
{
if(e.second == N)
return e.first;
}
}
};2.
https://leetcode.cn/problems/intersection-of-two-arrays/description/
cpp
class Solution {
public:
vector<int> intersection(vector<int>& nums1, vector<int>& nums2) {
// 用unordered_set对nums1中的元素去重
unordered_set<int> s1;
for (auto e : nums1)
s1.insert(e);
// 用unordered_set对nums2中的元素去重
unordered_set<int> s2;
for (auto e : nums2)
s2.insert(e);
// 遍历s1，如果s1中某个元素在s2中出现过，即为交集
vector<int> vRet;
for (auto e : s1)
{
if (s2.find(e) != s2.end())
vRet.push_back(e);
}
return vRet;
}
};3.
https://leetcode.cn/problems/first-unique-character-in-a-string/description/
cpp
class Solution {
public:
int firstUniqChar(string s) {
int count[26]={0};
for(auto ch:s)
{
count[ch-'a']++;
}
for(int i=0;i<s.size();i++)
{
if(count[s[i]-'a']==1)
{
return i;
}
}
return -1;
}
};
五.哈希的概念的讲解
1.闭散列
找位置又有两种方式:
a.线性探测
有没有可能找不到位置(位置被占满了),不会,后面我们会提出负载因子的概念
b.二次探测
c.其他规则
这里不展开讨论
2.开散列
六.哈希的大概实现
cpp
#pragma once
#include <vector>
enum State
{
EXIST,
EMPTY,
DELETE
};
template<class K, class V>
struct HashData
{
pair<K, V> _kv;
State _state = EMPTY;
};
template<class K, class V>
class HashTable
{
private:
vector<HashData<K, V>> _tables;
};