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# 1062.Longest-Repeating-Substring

## 1062. Longest Repeating Substring

## 题目地址

<https://leetcode.com/problems/longest-repeating-substring/>

## 题目描述

```
Given a string S, find out the length of the longest repeating substring(s). Return 0 if no repeating substring exists.

Example 1:
Input: "abcd"
Output: 0
Explanation: There is no repeating substring.

Example 2:
Input: "abbaba"
Output: 2
Explanation: The longest repeating substrings are "ab" and "ba", each of which occurs twice.

Example 3:
Input: "aabcaabdaab"
Output: 3
Explanation: The longest repeating substring is "aab", which occurs 3 times.

Example 4:
Input: "aaaaa"
Output: 4
Explanation: The longest repeating substring is "aaaa", which occurs twice.


Note:

The string S consists of only lowercase English letters from 'a' - 'z'.
1 <= S.length <= 1500
```

## 代码

### Approach #1 Binary Search + HashSet

```java
class Solution {
  public int longestRepeatingSubstring(String S) {
        int n = S.length();
    int left = 1;
    int right = n;
    int L;
    while (left <= right) {
      L = left + (right - left) / 2;
      if (search(L, n, S) == -1) {
           right = L - 1;
      } else {
        left = L + 1;
      }
    } 

    return left - 1;
  }

  private int search(int L, int n, String S) {
    HashSet<String> seen = new HashSet();
    String tmp;
    for (int start = 0; start < n - L + 1; start++) {
      tmp = S.substring(start, start + L);
      if (seen.contains(tmp))        return start;
      seen.add(tmp);
    }
    return -1;
  }
}
```