Reverse vowels in a given string

Given a string, your task is to reverse only the vowels of string.

Examples:

Input : hello
Output : holle

Input : hello world
Output : hollo werld

One simple solution is to store all the vowels while scanning the string and placing the vowels in the reverse order in another iteration of string.



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// C++ program to reverse order of vowels
#include<bits/stdc++.h>
using namespace std;
  
// utility function to check for vowel
bool isVowel(char c)
{
    return (c=='a' || c=='A' || c=='e' ||
            c=='E' || c=='i' || c=='I' ||
            c=='o' || c=='O' || c=='u' ||
            c=='U');
}
  
// Function to reverse order of vowels
string reverseVowel(string str)
{
    int j=0;
    // Storing the vowels separately
    string vowel;
    for (int i=0; str[i]!='\0'; i++)
        if (isVowel(str[i]))
            vowel[j++] = str[i];
  
    // Placing the vowels in the reverse
    // order in the string
    for (int i=0; str[i]!='\0'; i++)
        if (isVowel(str[i]))
            str[i] = vowel[--j] ;
  
    return str;
}
  
// Driver function
int main()
{
    string str = "hello world";
    cout << reverseVowel(str);
    return 0;
}

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Output:

hollo werld

Time complexity : O(n) where n = length of string
Auxiliary Space : O(v) where v = number of vowels in string

A better solution is to use two pointers scanning from beginning and end of the array respectively and manipulate vowels pointed by these pointers.

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// C++ program to reverse order of vowels
#include<bits/stdc++.h>
using namespace std;
  
// utility function to check for vowel
bool isVowel(char c)
{
    return (c=='a' || c=='A' || c=='e' ||
            c=='E' || c=='i' || c=='I' ||
            c=='o' || c=='O' || c=='u' ||
            c=='U');
}
  
// Function to reverse order of vowels
string reverseVowel(string str)
{
    // Start two indexes from two corners
    // and move toward each other
    int i = 0;
    int j = str.length()-1;
    while (i < j)
    {
        if (!isVowel(str[i]))
        {
            i++;
            continue;
        }
        if (!isVowel(str[j]))
        {
            j--;
            continue;
        }
  
        // swapping
        swap(str[i], str[j]);
  
        i++;
        j--;
    }
  
    return str;
}
  
// Driver function
int main()
{
    string str = "hello world";
    cout << reverseVowel(str);
    return 0;
}

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Output:

hollo werld

Time complexity : O(n) where n = length of string
Auxiliary Space : O(1)

This article is contributed by Gaurav Ahirwar and Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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