fix: handle odd-length palindromes by preserving middle character

3518/solution.cpp

@@ -9,6 +9,7 @@ public:
   std::string smallestPalindrome(std::string s, int k) {
 
     std::string half = s.substr(0, s.size() / 2);
+    char middle = (s.size() % 2 == 1) ? s[s.size() / 2] : '\0';
 
     std::ranges::sort(half);
 
@@ -28,11 +29,15 @@ public:
     }
 
     auto it = permutations.begin();
-    advance(it, k - 1);
+    std::advance(it, k - 1);
 
     std::string mirroredHalf = *it;
     std::ranges::reverse(mirroredHalf);
 
+    if (middle != '\0') {
+      return *it + std::string(1, middle) + mirroredHalf;
+    }
+
     return *it + mirroredHalf;
   }
 };