文章目录

• 第十五天
• • • 1.hash_map

第十五天

1.hash_map

前面说到了hash_map就类似于python中的dict词典，一个括号里的值，对应外面的一个值。
下面是一些基本用法

#include <map> // Header file to include map in the code
#include<iostream>
#include <string>
using namespace std;

int main() 
{
	map<int, string> mp; //creating a map
	map<int, string>::iterator it; //object for forloop
	  
	mp.insert(pair<int, string>(1, "Python"));  //inserting values
	mp.insert(pair<int, string>(2, "C")); 
	mp.insert(pair<int, string>(3, "Java"));
	mp.insert(pair<int, string>(4, "PHP"));
	mp.insert(pair<int, string>(5, "C++"));
	
	mp[6] = "Pearl"; //other way to add element in map
	mp[7] = "JavaScript";
	mp[8] = "R Programming";
	mp[9] = "Scala";
	mp[10] = "Rust";
	  
	std::cout<<endl<<"Map : "<<endl;
	for (it = mp.begin(); it != mp.end(); it++) {
	  std::cout<< it->first << " " << it->second << endl; //Printing the values after inserting
	}
	
	mp.erase(10); // removing 10th element
	mp.erase(9);  // removing 9th element
	std::cout<<endl<<"After removing 2 elements from the map"<<endl;
	
	for (it = mp.begin(); it != mp.end(); it++) {
	  std::cout<< it->first << " " << it->second << endl;
	}
	 	
	mp.erase(mp.find(6), mp.end()); //removing multiple elements from the map
	  
	std::cout<<endl<<"After removing multiple elements from the map"<<endl;
	
	for (it = mp.begin(); it != mp.end(); it++) {
	  std::cout<< it->first << " " << it->second << endl;
	}
	
	std::cout<<endl<<"Size of the map"<<mp.size(); //this will return the number of elements present in the map
	
	it = mp.find('c'); 
      
    if(it == mp.end()) 
        cout <<endl<< "Key-value pair not present in a map" ; 
    else
        cout <<endl<< "\nKey-value pair present : " 
          << it->first << "->" << it->second ; 
		 
    it = mp.find('SQL'); 
      
    if(it == mp.end()) 
        cout <<endl<< "\nKey-value pair not present in map" ; 
    else
        cout <<endl<< "\nKey-value pair present : " 
          << it->first << "->" << it->second ; 

	std::cout<< endl;
}

Output

Map :
1 Python
2 C
3 Java
4 PHP
5 C++
6 Pearl
7 JavaScript
8 R Programming
9 Scala
10 Rust

After removing 2 elements from the map
1 Python
2 C
3 Java
4 PHP
5 C++
6 Pearl
7 JavaScript
8 R Programming

After removing multiple elements from the map
1 Python
2 C
3 Java
4 PHP
5 C++

Size of the map5

Key-value pair present : 2->C

Key-value pair not present in a map

给定两个字符串 s 和 t ，编写一个函数来判断 t 是否是 s 的字母异位词。
注意：若 s 和 t 中每个字符出现的次数都相同，则称 s 和 t 互为字母异位词。

做起来很简单的，一遍出了，xs
第一种

class Solution {
public:
    bool isAnagram(string s, string t) {
        sort(s.begin(), s.end());
        sort(t.begin(), t.end());
        if (s == t)
            return true;
        return false;
    }
};

第二种，新学的hash_map

也是比较简单

class Solution {
public:
    bool isAnagram(string s, string t) {
        if (s.size() != t.size())
            return false;
        vector<int> dig(26);
        for (auto ch:s){
            dig[ch - 'a']++;
        }
        for (auto ch:t){
            dig[ch - 'a']--;
            if (dig[ch - 'a'] < 0)
                return false;
        }
        return true;
    }
};

给你一个字符串数组，请你将 字母异位词 组合在一起。可以按任意顺序返回结果列表。

字母异位词 是由重新排列源单词的字母得到的一个新单词，所有源单词中的字母都恰好只用一次。

示例 1:
输入: strs = [“eat”, “tea”, “tan”, “ate”, “nat”, “bat”]
输出: [[“bat”],[“nat”,“tan”],[“ate”,“eat”,“tea”]]

示例 2:
输入: strs = [""]
输出: [[""]]

示例 3:
输入: strs = [“a”]
输出: [[“a”]]

class Solution {
public:
    vector<vector<string>> groupAnagrams(vector<string>& strs) {
        unordered_map<string, vector<string>> mp;
        for (string& str: strs) {
            string key = str;
            sort(key.begin(), key.end());
            mp[key].emplace_back(str);
        }
        vector<vector<string>> ans;
        for (auto it = mp.begin(); it != mp.end(); ++it) {
            ans.emplace_back(it->second);
        }
        return ans;
    }
};

两数之和

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> hashtable;
        for (int i = 0; i < nums.size(); ++i) {
            auto it = hashtable.find(target - nums[i]);
            if (it != hashtable.end()) {
                return {it->second, i};
            }
            hashtable[nums[i]] = i;
        }
        return {};
    }
};