# Longest common substring

If you want to practice data structure and algorithm programs, you can go through 100+ data structure and algorithm programs.

In this post, we will see how to find the longest common substring in java.

## Program

Given two String, find longest common substring.
For example:

String 1: Java2blog
String 2: CoreJavaTutorial
Longest common subString is: Java

## Solution

### Brute force approach

You can solve this problem brute force. Let’s say you are given two String str1 and st2. Length of Str1 be m and length of str2 be n.You can find all substrings of str1 in o(m^2) time then search each of substring in str2, so total complexicity of algorithm will be o(m^2*n).

### Dynamic programming solution

You can solve this problem with the help of dynamic programming.Here is simple algorithm.

• Initialize 2D array of m*n named “dp”
• Iterate over str1 in outer loop(Using i)
• Iterate over str2 in inner loop(Using j)
• If str.charAt(i) == str2.charAt(j)
• If i or j=0 then put dp[i][j] = 1
• If i !=0 and j!=0
• dp[i][j] = dp[i-1][j-1] + 1
• Keep the track of max and endIndex in process
• Find substring with the help of endIndex and max.

Here max denotes the size of longest common substring.

Here is simple program to find longest common sustring in java.

When you run above program, you will get below output:

String 1: Java2blog
String 2: CoreJavaTutorial
Longest common subString is: Java

The time complexity of this algorithm is o(m*n)
That ‘s all about Longest common substring in java.

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