algorithm-class/大厂刷题班/class17/Code03_PalindromePairs2.java

package class17;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;

// 再次优化，不用哈希表，不用前缀树，用字符串哈希！
// 在算法周更班 : https://www.mashibing.com/live/1240
// 字符串哈希代码 : https://github.com/algorithmzuo/weekly-problems/blob/main/src/class_2023_05_5_week/Code04_StringHash.java
// 2023年5月第5周的课，看会！
// 本题测试链接 : https://leetcode.com/problems/palindrome-pairs/
// 字符串哈希 + Manacher算法！打败95%的人！
public class Code03_PalindromePairs2 {

	// 选一个质数做进制数
	public static int BASE = 499;

	// 计算一个字符串的哈希值
	public static long hashValue(String str) {
		if (str.equals("")) {
			return 0;
		}
		int n = str.length();
		long ans = str.charAt(0) - 'a' + 1;
		for (int j = 1; j < n; j++) {
			ans = ans * BASE + str.charAt(j) - 'a' + 1;
		}
		return ans;
	}

	// 字符串最大长度
	// 以下内容看字符串哈希的内容
	public static int MAXN = 301;

	public static long[] pow = new long[MAXN];

	static {
		pow[0] = 1;
		for (int j = 1; j < MAXN; j++) {
			pow[j] = pow[j - 1] * BASE;
		}
	}

	public static long[] hash = new long[MAXN];

	public static void buildHash(String str) {
		hash[0] = str.charAt(0) - 'a' + 1;
		for (int j = 1; j < str.length(); j++) {
			hash[j] = hash[j - 1] * BASE + str.charAt(j) - 'a' + 1;
		}
	}

	public static long hashValue(int l, int r) {
		if (l > r) {
			return 0;
		}
		long ans = hash[r];
		ans -= l == 0 ? 0 : (hash[l - 1] * pow[r - l + 1]);
		return ans;
	}

	public static List<List<Integer>> palindromePairs(String[] words) {
		HashMap<Long, Integer> hash = new HashMap<>();
		for (int i = 0; i < words.length; i++) {
			hash.put(hashValue(words[i]), i);
		}
		List<List<Integer>> res = new ArrayList<>();
		for (int i = 0; i < words.length; i++) {
			res.addAll(findAll(words[i], i, hash));
		}
		return res;
	}

	public static List<List<Integer>> findAll(String word, int index, HashMap<Long, Integer> hash) {
		List<List<Integer>> res = new ArrayList<>();
		String reverse = reverse(word);
		// 0代表""字符串
		Integer rest = hash.get(0L);
		if (rest != null && rest != index && word.equals(reverse)) {
			res.add(Arrays.asList(rest, index));
			res.add(Arrays.asList(index, rest));
		}
		if (!word.equals("")) {
			buildHash(reverse);
			int[] rs = manacherss(word);
			int mid = rs.length >> 1;
			for (int i = mid - 1; i >= 1; i--) {
				if (i - rs[i] == -1) {
					rest = hash.get(hashValue(0, mid - i - 1));
					if (rest != null && rest != index) {
						res.add(Arrays.asList(rest, index));
					}
				}
			}
			for (int i = mid + 1; i < rs.length; i++) {
				if (i + rs[i] == rs.length) {
					rest = hash.get(hashValue((mid << 1) - i, reverse.length() - 1));
					if (rest != null && rest != index) {
						res.add(Arrays.asList(index, rest));
					}
				}
			}
		}
		return res;
	}

	public static int[] manacherss(String word) {
		char[] mchs = manachercs(word);
		int[] rs = new int[mchs.length];
		int center = -1;
		int pr = -1;
		for (int i = 0; i != mchs.length; i++) {
			rs[i] = pr > i ? Math.min(rs[(center << 1) - i], pr - i) : 1;
			while (i + rs[i] < mchs.length && i - rs[i] > -1) {
				if (mchs[i + rs[i]] != mchs[i - rs[i]]) {
					break;
				}
				rs[i]++;
			}
			if (i + rs[i] > pr) {
				pr = i + rs[i];
				center = i;
			}
		}
		return rs;
	}

	public static char[] manachercs(String word) {
		char[] chs = word.toCharArray();
		char[] mchs = new char[chs.length * 2 + 1];
		int index = 0;
		for (int i = 0; i != mchs.length; i++) {
			mchs[i] = (i & 1) == 0 ? '#' : chs[index++];
		}
		return mchs;
	}

	public static String reverse(String str) {
		char[] chs = str.toCharArray();
		int l = 0;
		int r = chs.length - 1;
		while (l < r) {
			char tmp = chs[l];
			chs[l++] = chs[r];
			chs[r--] = tmp;
		}
		return String.valueOf(chs);
	}

}