modify code

大厂刷题班/class17/Code03_PalindromePairs1.java

@@ -4,8 +4,11 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 
+// 课上讲了哈希表+manacher
+// 有关manacher的解释，看这个帖子 : https://www.mashibing.com/question/detail/56727
+// 从代码层次讲了一下，例子非常详细
 // 测试链接 : https://leetcode.com/problems/palindrome-pairs/
-public class Code03_PalindromePairs {
+public class Code03_PalindromePairs1 {
 
 	public static List<List<Integer>> palindromePairs(String[] words) {
 		HashMap<String, Integer> wordset = new HashMap<>();
@@ -13,10 +16,7 @@ public class Code03_PalindromePairs {
 			wordset.put(words[i], i);
 		}
 		List<List<Integer>> res = new ArrayList<>();
-		//{ [6,23] 、 [7,13] }
 		for (int i = 0; i < words.length; i++) {
-			// i words[i]
-			// findAll(字符串，在i位置，wordset) 返回所有生成的结果返回
 			res.addAll(findAll(words[i], i, wordset));
 		}
 		return res;

大厂刷题班/class17/Code03_PalindromePairs2.java

@@ -0,0 +1,157 @@
+package class17;
+
+import java.util.ArrayList;
+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)) {
+			addRecord(res, rest, index);
+			addRecord(res, 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) {
+						addRecord(res, 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) {
+						addRecord(res, index, rest);
+					}
+				}
+			}
+		}
+		return res;
+	}
+
+	public static void addRecord(List<List<Integer>> res, int left, int right) {
+		List<Integer> newr = new ArrayList<>();
+		newr.add(left);
+		newr.add(right);
+		res.add(newr);
+	}
+
+	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);
+	}
+
+}