Queues, Deques & Priority Queues (Intro)

5. Practice Problems

1. Implement a queue using two stacks (amortised O(1)).
   দুটি stack দিয়ে queue বানান (amortised O(1))।
   ✨ Show Answer (উত্তর দেখুন)

   a1.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct QQ {
       stack<int> in, out;
       void push(int x) { in.push(x); }
       int  pop() {
           if (out.empty())
               while (!in.empty()) { out.push(in.top()); in.pop(); }
           int v = out.top(); out.pop(); return v;
       }
   };
   int main() {
       QQ q;
       for (int x : {1,2,3}) q.push(x);
       cout << q.pop() << " " << q.pop() << " " << q.pop();
   }

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2. Print the first negative integer in every window of size k.
   প্রতিটি size-k window-এ প্রথম negative integer প্রিন্ট করুন।
   ✨ Show Answer (উত্তর দেখুন)

   a2.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<int> a = {12,-1,-7,8,-15,30,16,28};
       int k = 3;
       deque<int> dq;
       for (int i = 0; i < (int)a.size(); i++) {
           if (a[i] < 0) dq.push_back(i);
           while (!dq.empty() && dq.front() <= i - k) dq.pop_front();
           if (i >= k - 1) cout << (dq.empty() ? 0 : a[dq.front()]) << " ";
       }
   }

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3. Rotten oranges (BFS). Given a grid of fresh (1) and rotten (2) oranges, return the minutes until all rot.
   Rotten oranges সমস্যা — BFS দিয়ে।
   ✨ Show Answer (উত্তর দেখুন)

   a3.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<vector<int>> g = {{2,1,1},{1,1,0},{0,1,1}};
       int R = g.size(), C = g[0].size(), fresh = 0, t = 0;
       queue<pair<int,int>> q;
       for (int i=0; i<R; i++) for (int j=0; j<C; j++) {
           if (g[i][j]==2) q.push({i,j});
           if (g[i][j]==1) fresh++;
       }
       int dx[]={-1,1,0,0}, dy[]={0,0,-1,1};
       while (!q.empty() && fresh) {
           int sz = q.size();
           while (sz--) {
               auto [x,y] = q.front(); q.pop();
               for (int d=0; d<4; d++) {
                   int nx=x+dx[d], ny=y+dy[d];
                   if (nx>=0&&nx<R&&ny>=0&&ny<C&&g[nx][ny]==1) {
                       g[nx][ny]=2; fresh--; q.push({nx,ny});
                   }
               }
           }
           t++;
       }
       cout << (fresh ? -1 : t);
   }

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4. Use a min-priority-queue to merge k sorted arrays into one.
   Min-PQ দিয়ে k সর্টেড অ্যারে merge করুন।
   ✨ Show Answer (উত্তর দেখুন)

   a4.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<vector<int>> arrs = {{1,5,9},{2,6},{3,4,10}};
       priority_queue<tuple<int,int,int>, vector<tuple<int,int,int>>, greater<>> pq;
       for (int i=0; i<(int)arrs.size(); i++) if (!arrs[i].empty()) pq.push({arrs[i][0], i, 0});
       while (!pq.empty()) {
           auto [v, i, j] = pq.top(); pq.pop();
           cout << v << " ";
           if (j+1 < (int)arrs[i].size()) pq.push({arrs[i][j+1], i, j+1});
       }
   }

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5. Reorganize a string so no two adjacent characters are the same (use a max-PQ on counts).
   পাশাপাশি একই char না হয় — string reorganize করুন।
   ✨ Show Answer (উত্তর দেখুন)

   a5.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "aaabbc";
       int cnt[26] = {};
       for (char c : s) cnt[c-'a']++;
       priority_queue<pair<int,char>> pq;
       for (int i=0; i<26; i++) if (cnt[i]) pq.push({cnt[i], 'a'+i});
       string out;
       while (pq.size() >= 2) {
           auto [c1, ch1] = pq.top(); pq.pop();
           auto [c2, ch2] = pq.top(); pq.pop();
           out += ch1; out += ch2;
           if (--c1) pq.push({c1, ch1});
           if (--c2) pq.push({c2, ch2});
       }
       if (!pq.empty()) {
           if (pq.top().first > 1) { cout << "impossible"; return 0; }
           out += pq.top().second;
       }
       cout << out;
   }

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6. Implement a stack using two queues (amortised).
   দুটি queue দিয়ে stack বানান।
   ✨ Show Answer (উত্তর দেখুন)

   a6.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct SS {
       queue<int> q;
       void push(int x) {
           q.push(x);
           int sz = q.size();
           while (--sz) { q.push(q.front()); q.pop(); }
       }
       int pop() { int v = q.front(); q.pop(); return v; }
   };
   int main() {
       SS s;
       for (int x : {1,2,3}) s.push(x);
       cout << s.pop() << " " << s.pop() << " " << s.pop();
   }

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7. Find the top-k frequent integers using a min-heap of size k.
   size-k min-heap দিয়ে top-k frequent integer বের করুন।
   ✨ Show Answer (উত্তর দেখুন)

   a7.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       vector<int> a = {1,1,1,2,2,3,4,4,4,4};
       int k = 2;
       unordered_map<int,int> cnt;
       for (int x : a) cnt[x]++;
       priority_queue<pair<int,int>, vector<pair<int,int>>, greater<>> pq;
       for (auto& [v, c] : cnt) {
           pq.push({c, v});
           if ((int)pq.size() > k) pq.pop();
       }
       while (!pq.empty()) { cout << pq.top().second << " "; pq.pop(); }
   }

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