Beginner · Strings

Reverse a String

Read a word and print it backwards. Your first taste of character arrays — the C-style string, which is just a char array with a hidden terminator.

At a glanceRead the word into a char array, get its length with strlen, and print the characters from last to first — a C string is just a char array ending in a hidden '\0'.

Problem

Input: One word (no spaces), up to 99 characters.

Output: The same characters, reversed.

Sample:

Input	Output
`hello`	`olleh`

Your solution

main.cpp — your solution

Custom input for "Run my input"

Judging runs in your browser against hidden tests. A runaway loop is cut off after 3 seconds — still, make sure every loop has an exit. Your code is saved in this browser as you type.

Stuck? Hints

Hint 1 — char array + strlen

char s[100]; cin >> s; reads one word. strlen(s) gives its length. Then loop from n-1 down to 0.

Hint 2 — Why stop at the terminator?

A C-style string ends with a zero byte. That is how functions know where the text ends.

Solution (try it first!)

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char s[100];
    cin >> s;
    int n = strlen(s);
    for (int i = n - 1; i >= 0; i--) cout << s[i];
    cout << endl;
    return 0;
}

It is the array cousin of Reverse a Number — same idea, different data type.

Deep dive

Why this works

A C-style string is an array plus a terminator. Reversing it in place with two indices — one from each end — is the pattern behind every in-place transform.

The cost

O(n) time, O(1) extra space in place; O(n) extra if you copy into a second array.

Where it goes wrong

Calling strlen in the loop condition makes the whole thing O(n²). Cache the length once.
Forgetting the terminator, or overwriting it, produces garbage past the end of the string.

Next step

The same array skills on characters: Count the Vowels.

Two things to remember

Two indices closing in from both ends: the in-place reversal pattern.
strlen inside a loop condition is a hidden O(n²).

🐞 This version fails — open it and try

Prints the array forward — never reverses

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char s[100];
    cin >> s;
    int n = strlen(s);
    for (int i = 0; i < n; i++) cout << s[i];
    cout << endl;
    return 0;
}