## LeetCode 1062. Longest Repeating Substring

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:

1. The string `S` consists of only lowercase English letters from `'a'` - `'z'`.
2. `1 <= S.length <= 1500`

Let dp[i][j] denotes S, up to i, and up to j<i, the number of common suffix.

When j and i are pointing to same char, dp[i][j] = dp[i-1][j-1]+1.

Maintain the maximum.

Time Complexity: O(n^2). n = S.length().

Space: O(n^2).

AC Java:

``` 1 class Solution {
2     public int longestRepeatingSubstring(String S) {
3         if(S == null || S.length() == 0){
4             return 0;
5         }
6
7         int n = S.length();
8         int res = 0;
9
10         int [][] dp = new int[n+1][n+1];
11         for(int i = 1; i<=n; i++){
12             for(int j = 1; j<i; j++){
13                 if(S.charAt(i-1) == S.charAt(j-1)){
14                     dp[i][j] = dp[i-1][j-1]+1;
15                     res = Math.max(res, dp[i][j]);
16                 }
17             }
18         }
19
20         return res;
21     }
22 }```