Top 25 C++ Interview Coding Questions

Carefully selected high-frequency C++ programs covering STL, OOP, algorithms, and memory concepts — designed for real interviews.

1.Swap two numbers using std::swap

#include 
using namespace std;
int main(){
    int a=5,b=10;
    swap(a,b);
    cout<



  
    
      2.Factorial using recursion
    
    
  
  #include 
using namespace std;
int fact(int n){
    if(n==0) return 1;
    return n*fact(n-1);
}
int main(){
    cout<



  
    
      3.Check prime number
    
    
  
  #include 
using namespace std;
int main(){
    int n=29,flag=1;
    for(int i=2;i<=n/2;i++)
        if(n%i==0){flag=0;break;}
    cout<<(flag?"Prime":"Not Prime");
}


  
    
      4.Reverse a string
    
    
  
  #include 
#include 
using namespace std;
int main(){
    string s="hello";
    reverse(s.begin(),s.end());
    cout<



  
    
      5.Palindrome number
    
    
  
  #include 
using namespace std;
int main(){
    int n=121,r=0,t=n;
    while(t){
        r=r*10+t%10;
        t/=10;
    }
    cout<<(n==r?"Palindrome":"Not Palindrome");
}


  
    
      6.GCD using STL
    
    
  
  #include 
#include 
using namespace std;
int main(){
    cout<<__gcd(24,36);
}


  
    
      7.Fibonacci series
    
    
  
  #include 
using namespace std;
int main(){
    int a=0,b=1,c,n=10;
    cout<



  
    
      8.Largest element in vector
    
    
  
  #include 
#include 
#include 
using namespace std;
int main(){
    vector v={3,8,1,9,4};
    cout<<*max_element(v.begin(),v.end());
}


  
    
      9.Second largest element
    
    
  
  #include 
#include 
#include 
using namespace std;
int main(){
    vector v={10,20,30,40};
    sort(v.begin(),v.end(),greater());
    cout<



  
    
      10.Bubble sort
    
    
  
  #include 
using namespace std;
int main(){
    int a[]={5,1,4,2},n=4;
    for(int i=0;ia[j+1])
                swap(a[j],a[j+1]);
    for(int i:a) cout<



  
    
      11.Binary search using STL
    
    
  
  #include 
#include 
using namespace std;
int main(){
    int a[]={1,3,5,7,9};
    cout<<(binary_search(a,a+5,7)?"Found":"Not Found");
}


  
    
      12.Sum of digits
    
    
  
  #include 
using namespace std;
int main(){
    int n=123,sum=0;
    while(n){ sum+=n%10; n/=10; }
    cout<



  
    
      13.String length
    
    
  
  #include 
using namespace std;
int main(){
    string s="hello";
    cout<



  
    
      14.Anagram check
    
    
  
  #include 
#include 
using namespace std;
int main(){
    string a="listen",b="silent";
    sort(a.begin(),a.end());
    sort(b.begin(),b.end());
    cout<<(a==b?"Anagram":"Not Anagram");
}


  
    
      15.Reverse linked list (concept)
    
    
  
  struct Node{
    int data;
    Node* next;
};


  
    
      16.Stack using STL
    
    
  
  #include 
#include 
using namespace std;
int main(){
    stack s;
    s.push(10); s.push(20);
    cout<



  
    
      17.Queue using STL
    
    
  
  #include 
#include 
using namespace std;
int main(){
    queue q;
    q.push(10); q.push(20);
    cout<



  
    
      18.Balanced parentheses
    
    
  
  #include 
#include 
using namespace std;
int main(){
    string s="(())";
    stack st;
    for(char c:s){
        if(c=='(') st.push(c);
        else if(!st.empty()) st.pop();
    }
    cout<<(st.empty()?"Balanced":"Not Balanced");
}


  
    
      19.Decimal to binary
    
    
  
  #include 
#include 
using namespace std;
int main(){
    cout<(10);
}


  
    
      20.Find duplicate element
    
    
  
  #include 
#include 
using namespace std;
int main(){
    int a[]={1,2,3,2};
    set s;
    for(int x:a){
        if(!s.insert(x).second)
            cout<



  
    
      21.Missing number
    
    
  
  #include 
using namespace std;
int main(){
    int a[]={1,2,4,5},n=5,sum=0;
    for(int i:a) sum+=i;
    cout<<(n*(n+1))/2 - sum;
}


  
    
      22.Array rotation
    
    
  
  #include 
#include 
using namespace std;
int main(){
    int a[]={1,2,3,4,5};
    rotate(a,a+2,a+5);
    for(int i:a) cout<



  
    
      23.Matrix transpose
    
    
  
  #include 
using namespace std;
int main(){
    int a[2][2]={{1,2},{3,4}};
    for(int i=0;i<2;i++){
        for(int j=0;j<2;j++)
            cout<



  
    
      24.Armstrong number
    
    
  
  #include 
using namespace std;
int main(){
    int n=153,sum=0,t=n;
    while(t){
        int d=t%10;
        sum+=d*d*d;
        t/=10;
    }
    cout<<(sum==n?"Armstrong":"Not Armstrong");
}


  
    
      25.Smart pointer example
    
    
  
  #include 
#include 
using namespace std;
int main(){
    unique_ptr p = make_unique(10);
    cout<<*p;
}