String Reversal Approaches in DSA C

Mental Model

Reversing a string means flipping its characters so the last becomes first and the first becomes last.

Analogy: Imagine reading a sentence backward, starting from the end and moving to the beginning, like rewinding a tape.

Original: h -> e -> l -> l -> o -> \nReversed: o -> l -> l -> e -> h -> \n↑start ↑end

Dry Run Walkthrough

Input: string: "hello"

Goal: Reverse the string so it reads "olleh"

Step 1: Swap first and last characters: 'h' and 'o'

o -> e -> l -> l -> h

Why: First and last characters must switch places to start reversal

Step 2: Swap second and second-last characters: 'e' and 'l'

o -> l -> l -> e -> h

Why: Continue swapping inward to reverse the string

Step 3: Middle character 'l' remains as is (odd length string)

o -> l -> l -> e -> h

Why: Middle character does not move in odd length strings

Result:

o -> l -> l -> e -> h
Reversed string: "olleh"

Annotated Code

DSA C

#include <stdio.h>
#include <string.h>

void reverseString(char *str) {
    int left = 0;
    int right = strlen(str) - 1;
    while (left < right) {
        // Swap characters at left and right
        char temp = str[left];
        str[left] = str[right];
        str[right] = temp;
        // Move pointers inward
        left++;
        right--;
    }
}

int main() {
    char str[] = "hello";
    reverseString(str);
    printf("%s\n", str);
    return 0;
}

while (left < right) {

loop until pointers meet or cross to cover all swaps

char temp = str[left]; str[left] = str[right]; str[right] = temp;

swap characters at left and right positions

left++; right--;

move pointers inward to next pair

Output

olleh

Complexity Analysis

Time: O(n) because each character is visited once during swapping

Space: O(1) because reversal is done in place without extra storage

vs Alternative: Compared to creating a new reversed string (O(n) space), this approach saves memory by swapping in place

Edge Cases

empty string

function returns immediately with no changes

DSA C

while (left < right) {

single character string

no swaps occur, string remains same

DSA C

while (left < right) {

string with all same characters

swapping identical characters keeps string unchanged

DSA C

char temp = str[left];

When to Use This Pattern

When asked to reverse a string or array in place, use two pointers moving inward swapping elements to achieve reversal efficiently.

Common Mistakes

Mistake: Swapping characters without moving pointers inward causes infinite loop
Fix: Increment left and decrement right pointers after each swap

Mistake: Using a new string without freeing memory in languages like C
Fix: Prefer in-place reversal to avoid extra memory management

Summary

Reverses a string by swapping characters from ends moving inward.

Use when you need to reverse strings efficiently without extra space.

The key is two pointers swapping characters until they meet in the middle.