Hash Tables: Functions, Collisions, Load Factor

8. Practice Problems

Each problem ships with a runnable C++ answer. Try first, peek later.

1. Two-Sum: given an array and a target, return indices of two numbers that add up to the target — in O(n).
   একটি array ও target দেওয়া আছে; এমন দুটি index ফেরত দিন যাদের যোগফল target — O(n) সময়ে।
   Show Answer (উত্তর দেখুন)

   two_sum.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<int> a = {2, 7, 11, 15};
       int target = 9;
       unordered_map<int, int> seen;
       for (int i = 0; i < (int)a.size(); ++i) {
           int need = target - a[i];
           if (seen.count(need)) {
               cout << seen[need] << " " << i << "\n";
               return 0;
           }
           seen[a[i]] = i;
       }
   }

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2. Group Anagrams: cluster strings whose letters are permutations of each other.
   যেসব string-এর অক্ষরগুলো একে অপরের permutation, সেগুলো একসাথে গ্রুপ করুন।
   Show Answer

   anagrams.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<string> v = {"eat","tea","tan","ate","nat","bat"};
       unordered_map<string, vector<string>> mp;
       for (auto& s : v) {
           string k = s;
           sort(k.begin(), k.end());
           mp[k].push_back(s);
       }
       for (auto& [k, g] : mp) {
           for (auto& w : g) cout << w << " ";
           cout << "\n";
       }
   }

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3. Longest substring without repeating characters — sliding window + hash map.
   পুনরাবৃত্তি ছাড়া সবচেয়ে দীর্ঘ substring বের করুন।
   Show Answer

   longest_uniq.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "abcabcbb";
       unordered_map<char, int> last;
       int best = 0, l = 0;
       for (int r = 0; r < (int)s.size(); ++r) {
           if (last.count(s[r]) && last[s[r]] >= l) l = last[s[r]] + 1;
           last[s[r]] = r;
           best = max(best, r - l + 1);
       }
       cout << best << "\n";
   }

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4. Number of contiguous subarrays whose sum equals K.
   এমন কয়টি contiguous subarray আছে যাদের যোগফল K-এর সমান?
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   subarray_sum_k.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<int> a = {1,2,3,-2,5};
       int K = 5, sum = 0, ans = 0;
       unordered_map<int, int> cnt; cnt[0] = 1;
       for (int x : a) {
           sum += x;
           ans += cnt[sum - K];
           cnt[sum]++;
       }
       cout << ans << "\n";
   }

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5. First non-repeating character in a stream of characters — print after each insertion.
   একটি character stream-এ প্রতিটি character যোগের পর প্রথম non-repeating character প্রিন্ট করুন।
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   first_unique.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "aabcbd";
       unordered_map<char, int> cnt;
       list<char> q;
       unordered_map<char, list<char>::iterator> pos;
       for (char c : s) {
           if (++cnt[c] == 1) { q.push_back(c); pos[c] = prev(q.end()); }
           else if (pos.count(c)) { q.erase(pos[c]); pos.erase(c); }
           cout << (q.empty() ? '#' : q.front()) << " ";
       }
       cout << "\n";
   }

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6. Design a HashSet<int> using only an array of vectors. Support add / remove / contains.
   শুধু array of vectors দিয়ে একটি HashSet<int> ডিজাইন করুন — add, remove, contains সাপোর্ট সহ।
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   my_hashset.cpp

   #include <bits/stdc++.h>
   using namespace std;
   class MySet {
       vector<vector<int>> b;
       int M = 1009;
   public:
       MySet() : b(M) {}
       void add(int k) { if (!contains(k)) b[k % M].push_back(k); }
       void remove(int k) {
           auto& v = b[k % M];
           v.erase(remove(v.begin(), v.end(), k), v.end());
       }
       bool contains(int k) {
           for (int x : b[k % M]) if (x == k) return true;
           return false;
       }
   };
   int main() {
       MySet s;
       s.add(5); s.add(1014); // 1014 % 1009 == 5 → collision
       cout << s.contains(5) << " " << s.contains(1014) << " " << s.contains(42) << "\n";
       s.remove(5);
       cout << s.contains(5) << "\n";
   }

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7. LRU Cache — implement get / put in O(1) using hash map + doubly linked list.
   hash map ও doubly linked list ব্যবহার করে O(1) সময়ে get / put-সহ একটি LRU cache বানান।
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   lru.cpp

   #include <bits/stdc++.h>
   using namespace std;
   class LRU {
       int cap;
       list<pair<int, int>> dq;
       unordered_map<int, list<pair<int, int>>::iterator> mp;
   public:
       LRU(int c) : cap(c) {}
       int get(int k) {
           if (!mp.count(k)) return -1;
           dq.splice(dq.begin(), dq, mp[k]);
           return mp[k]->second;
       }
       void put(int k, int v) {
           if (mp.count(k)) { mp[k]->second = v; dq.splice(dq.begin(), dq, mp[k]); return; }
           if ((int)dq.size() == cap) { mp.erase(dq.back().first); dq.pop_back(); }
           dq.push_front({k, v});
           mp[k] = dq.begin();
       }
   };
   int main() {
       LRU c(2);
       c.put(1,10); c.put(2,20);
       cout << c.get(1) << "\n";
       c.put(3,30); // evicts 2
       cout << c.get(2) << "\n";
   }

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