Reverse Vowels of a String
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Problem Statement Solution 1: Two-Pointer Approach Solution 2: Stack-Based Approach Solution 3: List Comprehension with Reverse Solution 4: Using Array Reverse Method Solution 5: In-Place Replacement with HashSet Solution 6: Iterative Replacement with Set Solution 7: Using Queue for Vowels Solution 8: Using Frequency Count and Replacements Solution 9: Using Character Frequency Mapping Solution 10: Custom Stack Implementation More Problems Solutions
Problem Statement
You have a string s that consists of both uppercase and lowercase letters. Your task is to reverse just the vowels in this string. The vowels are 'a', 'e', 'i', 'o', and 'u', and their case matters—so 'A' is different from 'a'. While reversing the vowels, the positions of the consonants and the overall structure of the string should stay exactly the same.
Example 1
Input: s = "hello"
Output: "holle"
Solution 1: Two-Pointer Approach
function reverseVowelsTwoPointer(s) {
const vowels = new Set('aeiouAEIOU');
const arr = s.split('');
let left = 0;
let right = arr.length - 1;
while (left < right) {
while (left < right && !vowels.has(arr[left])) left++;
while (left < right && !vowels.has(arr[right])) right--;
if (left < right) {
[arr[left], arr[right]] = [arr[right], arr[left]];
left++;
right--;
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsTwoPointer(s1); //output: holle
Solution 2: Stack-Based Approach
function reverseVowelsWithStack(s) {
const vowels = new Set('aeiouAEIOU');
const stack = [];
const arr = s.split('');
for (const char of arr) {
if (vowels.has(char)) stack.push(char);
}
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = stack.pop();
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsWithStack(s1); //output: holle
Solution 3: List Comprehension with Reverse
function reverseVowelsListComprehension(s) {
const vowels = new Set('aeiouAEIOU');
const arr = s.split('');
const vowelList = arr.filter(char => vowels.has(char)).reverse();
let vowelIndex = 0;
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = vowelList[vowelIndex++];
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsListComprehension(s1); //output: holle
Solution 4: Using Array Reverse Method
function reverseVowelsArrayReverse(s) {
const vowels = new Set('aeiouAEIOU');
const arr = s.split('');
const vowelIndices = [];
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
vowelIndices.push(i);
}
}
let left = 0;
let right = vowelIndices.length - 1;
while (left < right) {
const temp = arr[vowelIndices[left]];
arr[vowelIndices[left]] = arr[vowelIndices[right]];
arr[vowelIndices[right]] = temp;
left++;
right--;
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsArrayReverse(s1); //output: holle
Solution 5: In-Place Replacement with HashSet
function reverseVowelsInPlace(s) {
const vowels = new Set('aeiouAEIOU');
const arr = s.split('');
let i = 0, j = arr.length - 1;
while (i < j) {
if (!vowels.has(arr[i])) {
i++;
} else if (!vowels.has(arr[j])) {
j--;
} else {
[arr[i], arr[j]] = [arr[j], arr[i]];
i++;
j--;
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsInPlace(s1); //output: holle
Solution 6: Iterative Replacement with Set
function reverseVowelsIterative(s) {
const vowels = new Set('aeiouAEIOU');
const arr = s.split('');
let vowelChars = [];
for (const char of arr) {
if (vowels.has(char)) {
vowelChars.push(char);
}
}
let index = vowelChars.length - 1;
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = vowelChars[index--];
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsIterative(s1); //output: holle
Solution 7: Using Queue for Vowels
function reverseVowelsWithQueue(s) {
const vowels = new Set('aeiouAEIOU');
const queue = [];
const arr = s.split('');
for (const char of arr) {
if (vowels.has(char)) queue.push(char);
}
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = queue.pop();
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsWithQueue(s1); //output: holle
Solution 8: Using Frequency Count and Replacements
function reverseVowelsFrequency(s) {
const vowels = new Set('aeiouAEIOU');
const freq = [];
const arr = s.split('');
for (const char of arr) {
if (vowels.has(char)) freq.push(char);
}
freq.reverse();
let index = 0;
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = freq[index++];
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsFrequency(s1); //output: holle
Solution 9: Using Character Frequency Mapping
function reverseVowelsCharMap(s) {
const vowels = new Set('aeiouAEIOU');
const vowelChars = [];
const arr = s.split('');
for (const char of arr) {
if (vowels.has(char)) {
vowelChars.push(char);
}
}
vowelChars.reverse();
let charIndex = 0;
for (let i = 0; i < arr.length; i++) {
if (vowels.has(arr[i])) {
arr[i] = vowelChars[charIndex++];
}
}
return arr.join('');
}
const s1 = "hello";
reverseVowelsCharMap(s1); //output: holle
Solution 10: Custom Stack Implementation
function reverseVowelsCustomStack(s) {
const vowels = new Set('aeiouAEIOU');
const stack = [];
const arr = s.split('');
for (const char of arr) {
if (vowels.has(char)) {
stack.push(char);
}
}
return arr.map(char => {
if (vowels.has(char)) {
return stack.pop();
}
return char;
}).join('');
}
const s1 = "hello";
reverseVowelsCustomStack(s1); //output: holle