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// 214. Shortest Palindrome
#include "leetcode.h"
/*
* 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.
*/
char *shortestPalindrome(char *s) {
int n = strlen(s), dropped, right = 0, left = 0;
int *lut = (int *)malloc((n >> 1) * sizeof(int));
char *ans = (char *)malloc((n << 1) * sizeof(char));
if (n < 2)
return s;
lut[right++] = 0;
while (right < n >> 1) {
if (s[right] == s[left])
lut[right++] = ++left;
else if (!left)
lut[right++] = left;
else
left = lut[--left];
}
dropped = left = 0;
right = --n;
while (left < right)
if (s[left] == s[right]) {
left++;
right--;
} else if (left && lut[left - 1]) {
left = lut[--left];
dropped = n - right - left;
} else {
left = 0;
right--;
dropped = n - right;
}
free(lut);
for (left = 0; left < dropped; left++)
ans[left] = s[n - left];
for (left, right = 0; right <= n; left++, right++)
ans[left] = s[right];
ans[left] = '\0';
return ans;
}
int main() {
char *s1 = "aacecaaa", *s2 = "abcd";
printf("%s\n", shortestPalindrome(s1)); // expect: aaacecaaa
printf("%s\n", shortestPalindrome(s2)); // expect: dcbabcd
}
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