String Mechanics

A string is an array of chars with extras

string s = "loi choi";     // s[0] == 'l', s.size() == 8
for (char c : s) ...       // read-only traversal
for (char& c : s) c = toupper(c);   // in-place edit

char is a small integer: 'a' is 97, '0' is 48. So c - '0' converts a digit character to its value, and c - 'a' maps lowercase letters to 0..25 — the bridge into marking arrays for letters.

Building strings

string t;
t += c;          // amortized O(1) per char
t += "abc";      // append a literal
t.push_back('x'); // same as += for a single char

Repeated t = t + c in a loop is the classic slow version — each iteration copies the whole string, making the loop O(n^2). With n at 10^5 that is a TLE. Use +=.

Reading input

cin >> s reads one whitespace-delimited token. getline(cin, s) reads a whole line including spaces (and strips the newline). Mixing them is the classic beginner trap: after cin >> n, the newline stays in the buffer, so the next getline returns an empty string. Either use cin.ignore() after >> or read everything with getline.

Character categories

isdigit, isalpha, islower, isupper, tolower, toupper from <cctype>: they take an int (pass the char, cast if your warnings are pedantic) and return the classified/converted result. In contests these replace a hand-rolled if-chain, nothing more.