【题目】**214. 最短回文串
给定一个字符串 s，你可以通过在字符串前面添加字符将其转换为回文串。找到并返回可以用这种方式转换的最短回文串。
示例 1:
输入: "aacecaaa"
输出: "aaacecaaa"
示例 2:
输入: "abcd"
输出: "dcbabcd"
【解题思路1】字符串哈希
class Solution {
public String shortestPalindrome(String s) {
int n = s.length();
int base = 131, mod = 1000000007;
int left = 0, right = 0, mul = 1;
int best = -1;
for (int i = 0; i < n; ++i) {
left = (int) (((long) left * base + s.charAt(i)) % mod);
right = (int) ((right + (long) mul * s.charAt(i)) % mod);
if (left == right) {
best = i;
}
mul = (int) ((long) mul * base % mod);
}
String add = (best == n - 1 ? "" : s.substring(best + 1));
StringBuffer ans = new StringBuffer(add).reverse();
ans.append(s);
return ans.toString();
}
}
【解题思路2】KMP 算法
class Solution {
public String shortestPalindrome(String s) {
int n = s.length();
int[] fail = new int[n];
Arrays.fill(fail, -1);
for (int i = 1; i < n; ++i) {
int j = fail[i - 1];
while (j != -1 && s.charAt(j + 1) != s.charAt(i)) {
j = fail[j];
}
if (s.charAt(j + 1) == s.charAt(i)) {
fail[i] = j + 1;
}
}
int best = -1;
for (int i = n - 1; i >= 0; --i) {
while (best != -1 && s.charAt(best + 1) != s.charAt(i)) {
best = fail[best];
}
if (s.charAt(best + 1) == s.charAt(i)) {
++best;
}
}
String add = (best == n - 1 ? "" : s.substring(best + 1));
StringBuffer ans = new StringBuffer(add).reverse();
ans.append(s);
return ans.toString();
}
}