Linked Lists: Singly, Doubly, Circular

7. Practice Problems

1. Find the middle node of a linked list (for even count, return the second middle).
   Linked list-এর মাঝের node বের করুন (even হলে দ্বিতীয় middle)।
   ✨ Show Answer (উত্তর দেখুন)

   ans1.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   int main() {
       N* h = new N(1); N* t = h;
       for (int i = 2; i <= 6; i++) { t->nx = new N(i); t = t->nx; }
       N *s = h, *f = h;
       while (f && f->nx) { s = s->nx; f = f->nx->nx; }
       cout << s->v << "\n";
   }

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2. Merge two sorted linked lists into one sorted list.
   দুটি sorted linked list merge করে একটি sorted list বানান।
   ✨ Show Answer (উত্তর দেখুন)

   ans2.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   N* build(vector<int> a){N d(0);N* t=&d;for(int x:a){t->nx=new N(x);t=t->nx;}return d.nx;}
   int main() {
       N* a = build({1,3,5}); N* b = build({2,4,6,8});
       N dum(0); N* t = &dum;
       while (a && b) { if (a->v <= b->v) { t->nx = a; a = a->nx; } else { t->nx = b; b = b->nx; } t = t->nx; }
       t->nx = a ? a : b;
       for (N* c = dum.nx; c; c = c->nx) cout << c->v << " ";
       cout << "\n";
   }

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3. Detect a cycle and return the node where the cycle begins.
   Cycle detect করে যেই node থেকে cycle শুরু সেটিকে return করুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans3.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   int main() {
       N* a = new N(1); N* b = new N(2); N* c = new N(3); N* d = new N(4);
       a->nx = b; b->nx = c; c->nx = d; d->nx = b; // cycle starts at b
       N *s = a, *f = a;
       while (f && f->nx) { s = s->nx; f = f->nx->nx; if (s == f) break; }
       if (!f || !f->nx) { cout << "no cycle\n"; return 0; }
       s = a;
       while (s != f) { s = s->nx; f = f->nx; }
       cout << "cycle starts at " << s->v << "\n";
   }

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4. Remove duplicates from a sorted linked list.
   Sorted linked list থেকে duplicate সরিয়ে দিন।
   ✨ Show Answer (উত্তর দেখুন)

   ans4.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   int main() {
       N d(0); N* t = &d;
       for (int x : {1,1,2,3,3,4,5,5}) { t->nx = new N(x); t = t->nx; }
       for (N* c = d.nx; c && c->nx; ) {
           if (c->v == c->nx->v) c->nx = c->nx->nx;
           else c = c->nx;
       }
       for (N* c = d.nx; c; c = c->nx) cout << c->v << " ";
       cout << "\n";
   }

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5. Reverse a linked list in groups of size k.
   Linked list-কে k size-এর group-এ reverse করুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans5.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   N* revK(N* h, int k) {
       N* c = h; int cnt = 0;
       while (c && cnt < k) { c = c->nx; cnt++; }
       if (cnt < k) return h;
       N *prev = revK(c, k), *cur = h;
       for (int i = 0; i < k; i++) { N* nx = cur->nx; cur->nx = prev; prev = cur; cur = nx; }
       return prev;
   }
   int main() {
       N d(0); N* t = &d;
       for (int i = 1; i <= 7; i++) { t->nx = new N(i); t = t->nx; }
       N* h = revK(d.nx, 3);
       for (N* c = h; c; c = c->nx) cout << c->v << " ";
       cout << "\n";
   }

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6. Check if a linked list is a palindrome (O(n) time, O(1) space using reverse-half).
   Linked list palindrome কিনা — half reverse করে O(n) সময় ও O(1) space-এ যাচাই করুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans6.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   N* rev(N* h){N* p=nullptr;while(h){N* n=h->nx;h->nx=p;p=h;h=n;}return p;}
   int main() {
       N d(0); N* t = &d;
       for (int x : {1,2,3,2,1}) { t->nx = new N(x); t = t->nx; }
       N *s = d.nx, *f = d.nx;
       while (f && f->nx) { s = s->nx; f = f->nx->nx; }
       N* r = rev(s);
       N* a = d.nx; bool ok = true;
       while (r) { if (a->v != r->v) { ok = false; break; } a = a->nx; r = r->nx; }
       cout << (ok ? "YES" : "NO") << "\n";
   }

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7. Find intersection node of two linked lists (Y-shape).
   দুটি linked list-এর intersection node বের করুন (Y-shape)।
   ✨ Show Answer (উত্তর দেখুন)

   ans7.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   int main() {
       N* common = new N(8); common->nx = new N(9);
       N* A = new N(1); A->nx = new N(2); A->nx->nx = common;
       N* B = new N(5); B->nx = common;
       N *a = A, *b = B;
       while (a != b) {
           a = a ? a->nx : B;
           b = b ? b->nx : A;
       }
       cout << (a ? a->v : -1) << "\n";
   }

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8. Remove the n-th node from the end of a linked list in one pass.
   Linked list-এর শেষ থেকে n-তম node এক pass-এ সরিয়ে দিন।
   ✨ Show Answer (উত্তর দেখুন)

   ans8.cpp

   #include <bits/stdc++.h>
   using namespace std;
   struct N{int v;N* nx;N(int x):v(x),nx(nullptr){}};
   int main() {
       N d(0); N* t = &d;
       for (int i = 1; i <= 5; i++) { t->nx = new N(i); t = t->nx; }
       int n = 2;
       N *fast = &d, *slow = &d;
       for (int i = 0; i <= n; i++) fast = fast->nx;
       while (fast) { fast = fast->nx; slow = slow->nx; }
       slow->nx = slow->nx->nx;
       for (N* c = d.nx; c; c = c->nx) cout << c->v << " ";
       cout << "\n";
   }

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