LeetCode - Shortest Palindrome (KMP)

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"

.

思路：主要是要看从字符串的最左侧起的最长回文串是什么，可以借助kmp中最长公共前缀后缀的计算方法来计算

class Solution {
public:
string shortestPalindrome(string s) {
string r = s;
reverse(r.begin(), r.end());
string t = s + "#" + r;
int l = t.length();
vector<int> p(l, 0);
for(int i = 1; i < l; ++i){
int j = p[i - 1];
while(j > 0 && t[j] != t[i]) j = p[j - 1];
p[i] = j + (t[j] == t[i]);
}
return r.substr(0, r.size() - p[l - 1]) + s;
}
};