Reverse Vowels of a String — What I Missed, What I Learned (Two Pointer Reflection)

#programming #beginners #cpp #dsa

When learning the Two Pointer technique, some problems don’t fail completely — they fail quietly.

Reverse Vowels of a String was one of those problems for me.My code worked for many cases, but the logic had a hidden flaw.

This article explains:

My original approach
What I missed
The correction
What this problem taught me about two pointers

🧩 Problem: Reverse Vowels of a String

Description

You are given a string.
Reverse only the vowels while keeping all other characters in the same position.

Example

Input:  "leetcode"
Output: "leotcede"

✍️ My Initial Approach (Two Pointer)

I used two pointers:

left at the start
right at the end

My idea in English

If both characters are vowels → swap them

If left is not vowel → move left

Else → move right

This feels logical — and mostly works.

💻 My Original Code

bool isVowel(char c){
    if(tolower(c) == 'a' || tolower(c) == 'e' ||
       tolower(c) == 'i' || tolower(c) == 'o' ||
       tolower(c) == 'u')
        return true;
    return false;
}

int main() {
    string s = "leetcode";
    int left = 0, right = s.size() - 1;

    while(left < right){
        if(isVowel(s[left]) && isVowel(s[right])){
            swap(s[left], s[right]);
            left++;
            right--;
        }
        else if(!isVowel(s[left]) && isVowel(s[right]))
            left++;
        else
            right--;
    }

    cout << s;
}

⚠️ What I Missed (Important Insight)

❌ Missing Case

Note : What if both characters are NOT vowels?

In my code:

else always moves right-- . Even when both left and right are consonants
This causes:

Unbalanced pointer movement. Hard-to-detect logic bugs in edge cases

✅ Correct Pointer Movement Rule

We must handle all four cases clearly:

Left	Right	Action
vowel	vowel	swap, move both
not vowel	vowel	move left
vowel	not vowel	move right
not vowel	not vowel	move both

This makes pointer movement intentional, not accidental.

✅ Corrected Code (Clear & Safe)

while(left < right) {
    if(!isVowel(s[left])) {
        left++;
    }
    else if(!isVowel(s[right])) {
        right--;
    }
    else {
        swap(s[left], s[right]);
        left++;
        right--;
    }
}

This version: