題目

https://leetcode.com/problems/longest-duplicate-substring/

題解

這題暴力超時，看了 Related Topics，以及 Hint，主要用到 Rolling Hash，算法叫Rabin Karp，實現上參考：https://leetcode.com/problems/longest-duplicate-substring/discuss/1132925/JAVA-Solution

class Solution {public static final int MUL = 31;public String longestDupSubstring(String str) {int L = 0;int R = str.length();String result = "";while (L < R) {int M = (L + R) / 2; // M is lengthif (M == 0) break;HashSet<Long> seen = new HashSet<>();long hash = initHash(str.substring(0, M));seen.add(hash);long pow = 1;for (int l = 1; l < M; l++) pow *= MUL;for (int i = 1; i <= str.length() - M; i++) {hash = rollingHash(pow, hash, str, i, i + M - 1);if (seen.contains(hash)) {result = str.substring(i, i + M);}seen.add(hash);}if (result.length() == M) L = M + 1;else R = M;}return result;}public long initHash(String s) {long hash = 0;long mul = 1;for (int i = s.length() - 1; i >= 0; i--) {hash += s.charAt(i) * mul;mul *= MUL;}return hash;}public long rollingHash(long pow, long hash, String s, int i, int j) {return (hash - s.charAt(i - 1) * pow) * MUL + s.charAt(j);} }

總結

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