what is wrong in my logic reversing vowels in a string? - c++

Question

I tried solving a problem in leetcode
which asks the programmer to reverse the vowels in the given string.
When I wrote my code in C, it ran fine and passed all the test cases.
I tried writing the same code in C++ but for a particular test case, it failed.
bool isVowel(char a)
{
if(a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u')
return true;
if(a == 'A' || a == 'E' || a == 'I' || a == 'O' || a == 'U')
return true;
return false;
}
class Solution {
public:
string reverseVowels(string s) {
int i, j, k;
int len = s.length();
j = s.length() - 1;
i = 0;
k = 0;
string result;
//char result[len];
if (j < 0)
return s;
while(j >= 0) {
if (isVowel(s[j])) {
result[k] = s[j];
k++;
}
j--;
}
k = 0;
j = s.length() - 1;
while (i <= j) {
if(isVowel(s[i])) {
s[i] = result[k];
k++;
}
i++;
}
return s;
}
};
For some reason, when the input is "A new order began, a more Roman age bred Rowena." there is an error message AddressSanitizer: stack-buffer-overflow on address 0x7ffd4a543ab0 at pc 0x000000405efb.
When I tried to debug, I found that, the first while loop gets infinite. But when I replace the string result to char result[len], my code is working fine.
What is wrong in my approach?
Thanks
hago

Answer 1

You may not use the subscript operator for an empty string to change its value.
So your program has undefined behavior.
Pay attention to that in any case you are not reversing vowels in a string. You are trying to create a new string with reversed vowels from a given string. But this is not the same thing.
I can suggest the following Solution.:)
#include <iostream>
#include <string>
#include <cstring>
#include <cctype>
class Solution final
{
private:
static bool isVowel( char c )
{
const char *vowels = "AEIOU";
return std::strchr( vowels, std::toupper( static_cast<unsigned char>( c ) ) );
}
public:
static std::string & reverseVowels( std::string &s )
{
auto first = std::begin( s ), last = std::end( s );
do
{
while ( first != last && !isVowel( *first ) ) ++first;
if ( first != last )
{
while ( --last != first && !isVowel( *last ) );
}
if ( first != last ) std::iter_swap( first++, last );
} while ( first != last );
return s;
}
};
int main()
{
std::string s( "I am trying to write a program in C++" );
std::cout << s << '\n';
std::cout << Solution::reverseVowels( s ) << '\n';
return 0;
}
The program output is
I am trying to write a program in C++
i am tryong ta wreti o prigram In C++
Pay into account that the letter 'y' is not included in the set of vowels.

Answer 2

Your solution is correct but with a simple mistake.
When you declare string result; then this variable is declared with 0 size. So whenever you try to place character at some position (i.e result[0], result[1], ...) it finds that there is no allocated memory for this variable. So it throws error.
In stead of placing character to the result, you can add the character to this string.
So you can write result = result + s[j];
Code snap should be like this -
string result = "";
//char result[len];
if (j < 0)
return s;
while(j >= 0) {
if (isVowel(s[j])) {
result = result + s[j];
}
j--;
}
But adding character to a string takes more run-time.
Besides this, you can also use string.push_back() to add a single character to a string. It's complexity is overall O(n), n = length of the final string.
string result = "";
//char result[len];
if (j < 0)
return s;
while(j >= 0) {
if (isVowel(s[j])) {
result.push_back(s[j]);
}
j--;
}