LeetCode #214 Shortest Palindrome

Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given

"aacecaaa"

, return

"aaacecaaa"

.

Given

"abcd"

, return

"dcbabcd"

.
下面的代码是从Discuss中看到的，借用BM算法的思想，效率很高。

class Solution {
public:
string shortestPalindrome(string s) {
int n = s.length();
if (n < 2) return s;

string rs = s;
reverse(rs.begin(), rs.end());

int right[26];
for (int i = 0; i < n; i++)  right[s[i] - 'a'] = i;

int i = 0, j, tmp;
while (i < n) {
for (j = n - 1 - i; j >= 0; j--) {
if (s[j] != rs[i + j]) {
tmp = j - right[rs[i + j] - 'a'];
if (tmp <= 0) {
tmp = i + j + 1;
while (tmp < n && rs[tmp] != s[j]) tmp++;
tmp = tmp - i - j;
}
i += tmp;
break;
}
}
if (j < 0)  return rs.substr(0, i) + s;
}
return rs.substr(0, n - 1) + s;
}
};