Make palindromic string non-palindromic by rearranging its letters - c++

Question

Q: Make palindromic string non-palindromic by rearranging its letters.
I just want to know why my solution is failing (wrong answer) for some test cases when i submit the code. I am sure there is an easy solution such as sorting the whole string?
void makeNonPalindrome(string& s)
{
bool ans = false;
int l = s.length();
if(l % 2 == 0)
{
for(int i = l/2; i < l; i++)
{
if(s[l/2 - 1] != s[i])
{
swap(&s[l/2 - 1],&s[i]);
ans = true;
break;
}
}
if(!ans)
{
for(int i = 0; i < l/2-1; i++)
{
if(s[l/2 - 1] != s[i])
{
ans = true;
break;
}
}
}
}
else
{
for(int i = l/2 + 1; i < l; i++)
{
if(s[l/2 - 1] != s[i])
{
swap(&s[l/2 - 1],&s[i]);
ans = true;
break;
}
}
if(!ans)
{
for(int i = 0; i < l/2-1; i++)
{
if(s[l/2 - 1] != s[i])
{
ans = true;
break;
}
}
}
if(!ans)
{
if(s[l/2] != s[0])
{
swap(&s[l/2],&s[0]);
ans = true;
}
}
}
if(ans)
cout << s << '\n';
else
cout << -1 << '\n';
}

Answer 1

Rearranging a palindrome so it become non-palindromic can be done quite fast, by simply trying to swap two adjacent letters in the string if they are different.
For instance, in 'bob', you'd need to find the first distinct adjacent letters (that is b and o in our case), and swap them. The result would then be 'obb', which is not a palindrome.
void makeNonPalindrome(std::string& s) {
char tmp;
for (unsigned i = 0; i < s.length() - 1; i++) {
if (s[i] != s[i+1]) { // then swap s[i] and s[i+1]
tmp = s[i];
s[i] = s[i+1];
s[i+1] = tmp;
std::cout << s << '\n';
return;
}
}
std::cout << -1 << '\n';
}
This is a simpler way to make a palindrome non palindromic.
NB: this function assumes that the input is indeed a palindrome, so if you feed it a string like 'oob', it will output 'bob' which is a palindrome.

Answer 2

Given the input palindrome string s you can just use find_first_not_of to determine if the string can be rearranged into a non-palindrome at all, and if so what characters should be swapped to do this. For example:
const auto foo = s.find_first_not_of(s[0], 1);
If foo == string::npos it means that there isn't a possible non-palindrome rearrangement. Otherwise swap(s[0], s[foo]) will break the palindrome.
Live Example