PROGRAMMING:JMU DS longest common substring

Give 2 strings, output the longest common substring of 2 strings.
###Input format:
Enter 2 strings without spaces.
###Output format:
Outputs the longest common substring of 2 strings. If there is no common substring, output "null"
###Input sample 1:
```in
abcding
sbcdiofng
```
###Output sample 1:
```out
bcdi
```
###Input sample 2:
```
understanding
fw
```
###Output sample 2:
```
NULL
```






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