73. Reverse a String In-Place

#include <stdio.h>

void reverse_string(char *str) {
    int start = 0;
    int end = 0;

    // Find length of string
    while (str[end] != '\0') {
        end++;
    }
    end--;  // Move to last valid character

    // Swap characters from both ends
    while (start < end) {
        char temp = str[start];
        str[start] = str[end];
        str[end] = temp;
        start++;
        end--;
    }
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    int i = 0;
    while (str[i] != '\0') {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    reverse_string(str);
    printf("%s", str);
    return 0;
}

What is this about?

This problem uses pointer-based logic (or index swaps) to reverse characters in the same array, without extra memory.

Why it matters in firmware?

• Low-level buffer manipulation often needs reversal (e.g., endian swap)
• Memory is limited; in-place logic avoids extra allocation
• Good practice for boundary indexing and pointer logic

Solution Logic

• Find end of string
• Swap characters from start ↔ end
• Move pointers inward until they meet