unordered_set

unordered_set uses a hash table to store and retrieve its elements. 
This makes unordered_set faster for lookups, insertions, and deletions in most cases, but it does not preserve the order of elements.

hwo to create a unordered_set in c++?


//////////////c++///////////////
#include <unordered_set>
using namespace std;
unordered_set<int> mySet;
//////////////c++///////////////


This creates an empty unordered_set of integers named mySet.

unordered_set provides several member functions for adding, removing, and searching for elements, including:

insert(): Adds an element to the set
erase(): Removes an element from the set
clear(): Removes all elements from the set
find(): Searches for an element in the set
size(): Returns the number of elements in the set


//////////////c++///////////////
#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
  unordered_set<string> words = {"apple", "banana", "cherry", "banana"};

  cout << "The set contains " << words.size() << " elements:" << endl;
  for (const string& word : words) {
    cout << "  " << word << endl;
  }

  if (words.find("banana") != words.end()) {
    cout << "The set contains 'banana'." << endl;
  } else {
    cout << "The set does not contain 'banana'." << endl;
  }

  return 0;
}
//////////////c++///////////////

Leetcode 128 Longest Consecutive Sequence

//////////////c++///////////////
class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        unordered_set<int> q;
        for (int i = 0; i < nums.size(); i++) {
            q.insert(nums[i]);
        }
        int ans = 0;
        while (!q.empty()) {
            int now = *q.begin();
            q.erase(now);
            int l = now - 1, r = now + 1;
            while (q.find(l) != q.end()) {
                q.erase(l);
                l--;
            }
            while(q.find(r) != q.end()) {
                q.erase(r);
                r++;
            }
            l = l + 1, r = r - 1;
            ans = max(ans, r - l + 1);
        }
        return ans;
    }
};
//////////////c++///////////////