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LongestPalindrome.java
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LongestPalindrome.java
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/*
* Copyright (c) [2017] [Haibo(Tristan) Yan]
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.haibo.yan.algorithm.array;
import java.util.stream.IntStream;
/**
* Manacher Algorithm to calculate longest Palindrome
*
* See https://en.wikipedia.org/wiki/Longest_palindromic_substring, this only works for no $ in the string, we may be
* add randomize character as the separator.
*
*/
public class LongestPalindrome {
public int lenOfLongest(char[] cs) {
if (cs.length <= 1) {
return cs.length;
}
char[] oddCs = new char[2 * cs.length + 1];
oddCs[0] = '$';
for (int i = 0; i < cs.length; i++) {
oddCs[2 * i + 1] = cs[i];
oddCs[2 * i + 2] = '$';
}
return IntStream.of(plenWithOddArray(oddCs)).max().getAsInt() / 2;
}
private int[] plenWithOddArray(char[] cs) {
int[] pInts = new int[cs.length];
int i = 0;
while (i < cs.length) {
pInts[i] = plen(cs, i);
int j = 1;
for (; j <= (pInts[i] >> 1); j++) {
int leftJ = i - j - (pInts[i - j] >> 1);
int leftI = i - (pInts[i] >> 1);
if (leftJ < leftI) {
pInts[i + j] = pInts[i - j];
} else if (leftJ > leftI) {
pInts[i + j] = 1 + 2 *((pInts[i] >> 1) - j);
} else {
break;
}
}
i = i + j;
}
return pInts;
}
private int plen(char[] cs, int position) {
int len = 1;
int i = position - 1, j = position + 1;
while (i >= 0 && j < cs.length && cs[i] == cs[j]) {
len += 2;
i--;
j++;
}
return len;
}
}