Reverse Vowels of a String

LeetCode 345 | Difficulty: Easy

Easy

Problem Description

Given a string s, reverse only all the vowels in the string and return it.

The vowels are 'a', 'e', 'i', 'o', and 'u', and they can appear in both lower and upper cases, more than once.

Example 1:

Input: s = "IceCreAm"

Output: "AceCreIm"

Explanation:

The vowels in s are ['I', 'e', 'e', 'A']. On reversing the vowels, s becomes "AceCreIm".

Example 2:

Input: s = "leetcode"

Output: "leotcede"

Constraints:

- `1 <= s.length <= 3 * 10^5`

- `s` consist of **printable ASCII** characters.

Topics: Two Pointers, String

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Approach

Direct Approach

This problem can typically be solved with straightforward iteration or simple data structure usage. Focus on correctness first, then optimize.

When to use

Basic problems that test fundamental programming skills.

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Solutions

Solution 1: C# (Best: 136 ms)

Metric	Value
Runtime	136 ms
Memory	N/A
Date	2018-04-05

Solution

public class Solution {
    public string ReverseVowels(string s) {
       char[] sArray = s.ToCharArray();
    int len = s.Length;
    if(String.IsNullOrEmpty(s) || len==1) return s;
    int i=0, j=len-1;
    while(i<j)
    {
        while(!IsVowel(sArray[i]) && i<j)
        {
            i++;
        }
        while(!IsVowel(sArray[j]) && i<j)
        {
            j--;
        }
        if(i<j)
        {
            char temp = sArray[i];
            sArray[i] = sArray[j];
            sArray[j] = temp;
        }
        i++;
        j--;
    }
    return new string(sArray);
}

public bool IsVowel(char c)
{
    char[] vowels = {'A','E','I','O','U','a','e','i','o','u'};
    return vowels.Contains(c);
}
}

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Complexity Analysis

Approach	Time	Space
Two Pointers	$O(n)$	$O(1)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.