Strings & Pattern Basics

8. Practice Problems

1. Count the number of vowels in a string.
   একটি string-এ vowel-এর সংখ্যা গুনুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans1.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "Bangladesh University of Engineering and Technology";
       int c = 0;
       for (char ch : s) {
           char x = tolower(ch);
           if (x=='a'||x=='e'||x=='i'||x=='o'||x=='u') c++;
       }
       cout << c << "\n";
   }

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2. Reverse the words in a sentence (e.g., "I love DSA" → "DSA love I").
   একটি বাক্যের শব্দগুলো reverse করুন (যেমন "I love DSA" → "DSA love I")।
   ✨ Show Answer (উত্তর দেখুন)

   ans2.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "I love DSA from Dhaka";
       stringstream ss(s);
       vector<string> w; string tok;
       while (ss >> tok) w.push_back(tok);
       reverse(w.begin(), w.end());
       for (int i = 0; i < (int)w.size(); i++) cout << w[i] << (i+1<(int)w.size()?" ":"\n");
   }

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3. Check if a string is a palindrome ignoring spaces, punctuation and case.
   Space, punctuation ও case উপেক্ষা করে palindrome কিনা যাচাই করুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans3.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "A man, a plan, a canal: Panama";
       int l = 0, r = s.size() - 1;
       bool ok = true;
       while (l < r) {
           while (l < r && !isalnum(s[l])) l++;
           while (l < r && !isalnum(s[r])) r--;
           if (tolower(s[l]) != tolower(s[r])) { ok = false; break; }
           l++; r--;
       }
       cout << (ok ? "YES" : "NO") << "\n";
   }

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4. Find the longest palindromic substring (brute-force O(n³)).
   Longest palindromic substring বের করুন (brute-force O(n³))।
   ✨ Show Answer (উত্তর দেখুন)

   ans4.cpp

   #include <bits/stdc++.h>
   using namespace std;
   bool isPal(const string& s,int l,int r){while(l<r){if(s[l++]!=s[r--]) return false;}return true;}
   int main() {
       string s = "babad", best = "";
       int n = s.size();
       for (int i = 0; i < n; i++)
           for (int j = i; j < n; j++)
               if (isPal(s, i, j) && j - i + 1 > (int)best.size())
                   best = s.substr(i, j - i + 1);
       cout << best << "\n";
   }

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5. Find the first non-repeating character in a string.
   String-এ প্রথম non-repeating character খুঁজুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans5.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string s = "leetcode";
       int f[256] = {0};
       for (char c : s) f[(int)c]++;
       char ans = '?';
       for (char c : s) if (f[(int)c] == 1) { ans = c; break; }
       cout << ans << "\n";
   }

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6. Check whether two strings are anagrams using a 26-int frequency array.
   26-int frequency array দিয়ে দুটি string anagram কিনা যাচাই করুন।
   ✨ Show Answer (উত্তর দেখুন)

   ans6.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int main() {
       string a = "anagram", b = "nagaram";
       if (a.size() != b.size()) { cout << "NO\n"; return 0; }
       int f[26] = {0};
       for (char c : a) f[c-'a']++;
       for (char c : b) f[c-'a']--;
       bool ok = true;
       for (int i = 0; i < 26; i++) if (f[i]) ok = false;
       cout << (ok ? "YES" : "NO") << "\n";
   }

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7. Implement strStr (return index of first occurrence of needle in haystack, or -1).
   strStr বানান — haystack-এ needle প্রথম যেখানে আছে সেই index, না থাকলে -1।
   ✨ Show Answer (উত্তর দেখুন)

   ans7.cpp

   #include <bits/stdc++.h>
   using namespace std;
   int strStr(const string& H, const string& N) {
       int n = H.size(), m = N.size();
       if (m == 0) return 0;
       for (int i = 0; i + m <= n; i++) {
           int j = 0;
           while (j < m && H[i+j] == N[j]) j++;
           if (j == m) return i;
       }
       return -1;
   }
   int main() {
       cout << strStr("sadbutsad", "sad") << "\n";
       cout << strStr("hello", "world") << "\n";
   }

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