string s1;                    // empty string
string s2 = "hello";          // from C-string literal
string s3(5, 'a');            // "aaaaa"
string s4 = s2.substr(1, 3);  // "ell"
string s5(s2);                // copy of s2

s.length();       // number of characters (use s.size() interchangeably)
s.empty();        // true if length is 0
s.clear();        // makes the string empty

// Access
s[0];             // first character (no bounds checking!)
s.at(0);          // first character (throws if out of bounds)
s.front();        // first character
s.back();         // last character

// Modification
s.push_back('x'); // append character
s += " world";    // append string
s.insert(2, "!!");       // insert at position 2
s.erase(1, 3);           // erase 3 characters starting at position 1
s.replace(0, 2, "Hi");   // replace 2 chars at position 0 with "Hi"

// Search
s.find("lo");           // index of first occurrence (string::npos if not found)
s.rfind("l");           // index of last occurrence
s.find_first_of("aeiou"); // index of first vowel
s.find_first_not_of(" "); // index of first non-space

// Comparison
s1 == s2;         // equality
s1 != s2;         // inequality
s1 < s2;          // lexicographic comparison
s1.compare(s2);   // returns 0 if equal, negative if s1 < s2, positive if s1 > s2

// Range-based for (read-only)
for (char c : s) {
    cout << c;
}

// Range-based for (modifiable)
for (char &c : s) {
    c = toupper(c);
}

// Index-based
for (int i = 0; i < (int)s.size(); i++) {
    cout << s[i];
}

// Number to string
string s = to_string(42);      // "42"
string s = to_string(3.14);    // "3.140000"

// String to number
int n = stoi("123");           // 123
long long ll = stoll("123456789012345"); // 123456789012345
double d = stod("3.14");       // 3.14

// Lowercase to uppercase
char upper = lowercase - 32;  // or: toupper(lowercase)

// Uppercase to lowercase
char lower = uppercase + 32;  // or: tolower(uppercase)

// Character to digit
int d = c - '0';  // '5' - '0' = 5

// Digit to character
char c = d + '0';  // 5 + '0' = '5'

#include <cctype>

isalpha(c);   // true if letter
isdigit(c);   // true if digit
isalnum(c);   // true if letter or digit
isspace(c);   // true if whitespace
isupper(c);   // true if uppercase
islower(c);   // true if lowercase
toupper(c);   // converts to uppercase
tolower(c);   // converts to lowercase

int freq[26] = {};  // for lowercase letters
for (char c : s) {
    freq[c - 'a']++;
}

// Check if all characters are unique
bool unique = true;
for (int i = 0; i < 26; i++) {
    if (freq[i] > 1) { unique = false; break; }
}

bool isPalindrome(const string &s) {
    int n = s.size();
    for (int i = 0; i < n / 2; i++) {
        if (s[i] != s[n - 1 - i]) return false;
    }
    return true;
}