214. Shortest Palindrome

You are given a string s. You can convert s to a palindrome by adding characters in front of it.

Return the shortest palindrome you can find by performing this transformation.
 

Example 1:

Input: s = “aacecaaa”
Output: “aaacecaaa”.

Example 2:

Input: s = “abcd”
Output: “dcbabcd”

Constraints:

• $0<=s.length<=5\ast 10^4$
• s consists of lowercase English letters only.

From: LeetCode
Link: 214. Shortest Palindrome

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Solution:

Ideas:

1. Reverse the input string: This will help us find the longest palindromic prefix.
2. Find the longest palindromic prefix: By comparing the original string with the suffixes of the reversed string, we determine the longest prefix of the original string that is also a suffix of the reversed string.
3. Form the result string: Add the necessary characters (the part of the reversed string that does not match the prefix) in front of the original string to make it a palindrome.

Code:

void reverseString(char* str) {int n = strlen(str);for (int i = 0; i < n / 2; i++) {char temp = str[i];str[i] = str[n - i - 1];str[n - i - 1] = temp;}
}char* shortestPalindrome(char* s) {int n = strlen(s);if (n == 0) return "";// Create the reversed stringchar* rev_s = (char*)malloc((n + 1) * sizeof(char));strcpy(rev_s, s);reverseString(rev_s);// Find the longest palindromic prefixint i;for (i = n; i >= 0; i--) {if (strncmp(s, rev_s + n - i, i) == 0) {break;}}// Build the shortest palindrome by adding the necessary characters in front of schar* result = (char*)malloc((2 * n - i + 1) * sizeof(char));strcpy(result, rev_s);strncat(result, s + i, n - i);// Free allocated memoryfree(rev_s);return result;
}