Problem Solving
Beginner to Advanced

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n; cin>>n;
    cout << ((n&1) ? "Odd" : "Even") << "\n";
}

int a=5, b=9;

// Method 1: temp (safest, use this)
int temp=a; a=b; b=temp;

// Method 2: XOR (no extra space)
a^=b; b^=a; a^=b;

// Method 3: STL (best in real code)
swap(a,b);

int sumDigits(int n){
    n = abs(n);         // handle negatives
    int sum=0;
    while(n>0){
        sum += n%10;    // add last digit
        n   /= 10;      // remove last digit
    }
    return sum;
}

int reverseDigits(int n){
    bool neg = (n<0); n=abs(n);
    long long rev=0;           // long long avoids overflow
    while(n>0){
        rev = rev*10 + n%10;
        n  /= 10;
    }
    return neg ? -rev : rev;
}

int countDigits(int n){
    if(n==0) return 1;
    n = abs(n);
    int cnt=0;
    while(n>0){ cnt++; n/=10; }
    return cnt;
}
// Math:   (int)log10(abs(n)) + 1  [n != 0]
// String: to_string(abs(n)).size()

long long a,b,c; cin>>a>>b>>c;
cout << (a%c + b%c) % c << "\n";          // (a+b) mod c
cout << (a%c * (b%c)) % c << "\n";        // (a*b) mod c

int gcd(int a, int b){
    return b==0 ? a : gcd(b, a%b);
}
int lcm(int a, int b){
    return a/gcd(a,b)*b;    // divide first to avoid overflow
}
// C++17 built-ins: __gcd(a,b)  |  std::lcm(a,b) from <numeric>

bool isPrime(int n){
    if(n<2) return false;
    if(n==2) return true;
    if(n%2==0) return false;
    for(int i=3; i*i<=n; i+=2)   // odd divisors only
        if(n%i==0) return false;
    return true;
}

int countDivisors(int n){
    int cnt=0;
    for(int i=1; i*i<=n; i++){
        if(n%i==0){
            cnt++;                   // i is a divisor
            if(i != n/i) cnt++;      // n/i is another; skip if i==n/i
        }
    }
    return cnt;
}

bool isPerfectSquare(int n){
    if(n<0) return false;
    int r = (int)sqrt(n);
    return (r*r==n) || (r+1)*(r+1)==n;  // guard FP error
}

int cubeRoot(int n){
    int lo=0, hi=n;
    while(lo<=hi){
        long long mid = lo + (hi-lo)/2;
        long long cube = mid*mid*mid;
        if(cube == n) return mid;
        else if(cube < n) lo = mid+1;
        else hi = mid-1;
    }
    return hi;   // floor cube root
}

double simpleInterest(double P, double R, double T){
    return (P * R * T) / 100.0;
}

double euclidDist(double x1,double y1,double x2,double y2){
    return hypot(x2-x1, y2-y1);  // hypot handles overflow safely
}

long long l,b,h; cin>>l>>b>>h;
cout << "Volume = " << l*b*h << "\n";
cout << "TSA    = " << 2*(l*b + b*h + h*l) << "\n";
cout << "LSA    = " << 2*h*(l+b) << "\n";

bool isBinary(long long n){
    if(n==0) return true;
    while(n>0){
        int d = n%10;
        if(d!=0 && d!=1) return false;
        n/=10;
    }
    return true;
}
// 1011 → digits 1,1,0,1 → all valid → true

// Print binary (skip leading zeros)
void printBinary(int n){
    bool leading=true;
    for(int i=31;i>=0;i--){
        int bit=(n>>i)&1;
        if(bit) leading=false;
        if(!leading) cout<<bit;
    }
    if(leading) cout<<0;  // n=0 edge case
    cout<<"\n";
}

// Bit shifts
int doubled = n << 1;   // n * 2
int halved  = n >> 1;   // n / 2 (floor)

// Cleaner: bitset prints fixed 8 bits
cout << bitset<8>(n) << "\n";

bool isLeap(int y){
    return (y%400==0) || (y%100!=0 && y%4==0);
}

bool isSparse(int n){
    return (n & (n>>1)) == 0;
}

int a,b,c; cin>>a>>b>>c;

// Method 1: if-else (recommended for readability)
int greatest;
if(a>=b && a>=c)   greatest=a;
else if(b>=c)       greatest=b;
else                greatest=c;

// Method 2: STL max (cleanest)
greatest = max({a,b,c});

cout << greatest << "\n";

bool isVowel(char c){
    c = tolower(c);
    return c=='a'||c=='e'||c=='i'||c=='o'||c=='u';
}
// Or: string("aeiou").find(tolower(c)) != string::npos

// Single character
char c='a';
cout << (char)toupper(c) << "\n";    // 'A'
cout << (char)(c^32) << "\n";       // 'A' (XOR toggle)

// Entire string
string s = "Hello World";
transform(s.begin(), s.end(), s.begin(), ::toupper);
// s is now "HELLO WORLD"

int oppositeFace(int top){ return 7 - top; }

int replace0with5(int n){
    if(n==0) return 5;
    int result=0, place=1;
    while(n>0){
        int d = n%10;
        if(d==0) d=5;
        result += d * place;
        place *= 10;
        n    /= 10;
    }
    return result;
}

long long sumN(int n){
    return (long long)n*(n+1)/2;
}
long long sumOfSums(int n){
    return (long long)n*(n+1)*(n+2)/6;
}

int n; cin>>n;
for(int i=1; i<=10; i++)
    cout << n << " x " << i << " = " << n*i << "\n";

void fibonacci(int n){
    long long a=0, b=1;
    for(int i=0; i<n; i++){
        cout << a << " ";
        long long next = a+b;
        a = b;
        b = next;
    }
    cout << "\n";
}

double apSum(double a, double d, int n){
    return n/2.0 * (2*a + (n-1)*d);
}
double gpSum(double a, double r, int n){
    if(r==1.0) return a*n;
    return a * (pow(r,n)-1.0) / (r-1.0);
}

long long triangular(int n){
    return (long long)n*(n+1)/2;
}

// Mean
double mean(vector<int>& a){
    return accumulate(a.begin(),a.end(),0.0) / a.size();
}

// Median
double median(vector<int> a){
    sort(a.begin(),a.end());
    int n=a.size();
    return (n%2) ? a[n/2] : (a[n/2-1]+a[n/2])/2.0;
}

// Running average (no array needed)
void runningAvg(){
    double avg=0; int cnt=0, x;
    while(cin>>x){
        cnt++;
        avg += (x - avg) / cnt;
        cout << fixed << setprecision(2) << avg << "\n";
    }
}

bool isFascinating(int n){
    string s = to_string(n) + to_string(2*n) + to_string(3*n);
    if(s.size() != 9) return false;
    sort(s.begin(), s.end());
    return s == "123456789";
}
// Only 192, 219, 273, 327, 192... (few exist below 1000)