chore(CPlusPlus): adding longest common substring (#389)

Co-authored-by: Your Name <you@example.com>
Co-authored-by: Ming Tsai <37890026+ming-tsai@users.noreply.github.com>

algorithms/CPlusPlus/Dynamic-Programming/longest-common-substring.cpp

@@ -0,0 +1,48 @@
+//Given two strings ‘s1’ and ‘s2’, find the length of the longest common substring.
+//NOTE: A substring is a contiguous part of string.
+// Input : X = “abcdxyz”, y = “xyzabcd” 
+// Output : 4 
+// Explanation: The longest common substring is “abcd” and is of length 4.
+
+//Program using Dynamic Programming
+//TIME COMPLEXITY: O(m*n) 
+#include <iostream>
+using namespace std;
+int longestCommonSubstring(string s1,string s2,int n,int m)
+{
+  int ans=0;
+  int T[n+1][m+1];
+  for(int i=0;i<=n;i++)
+  {
+    for(int j=0;j<=m;j++)
+    {
+      if(i==0 || j==0) // if length of any of the string is 0 then the length of longest common substring will be 0 
+      T[i][j]=0;
+      else
+      if(s1[i-1]==s2[j-1]) // if characters are equal, add 1 and update the answer according to maximum length
+      {
+        T[i][j]=1+T[i-1][j-1];
+        ans=max(ans,T[i][j]);
+      }
+      else//if characters are not equal, make the current value in the array as 0
+      T[i][j]=0;
+    }
+  }
+  return ans;
+}
+int main()
+{
+  string s1,s2;
+  cout<<"Enter 1st string: "<<endl;
+  cin>>s1;
+  cout<<"Enter 2nd string: "<<endl;
+  cin>>s2;
+  int n=s1.length();
+  int m=s2.length();
+  cout<<"Length of longest common substring: "<<longestCommonSubstring(s1,s2,n,m);
+  return 0;
+}
+
+// s1: abcde
+// s2:abfce
+// output: 2(ab)

algorithms/CPlusPlus/README.md

@@ -16,8 +16,8 @@
 ## Dynamic-Programming
 
 1. [Longest Common Subsequence](Dynamic-Programming/longest-common-subsequence.cpp)
-2. [0/1-knapsack](Dynamic-Programming/01-knapsack-bottom-up.cpp)
-
+2. [Longest Common Substring](Dynamic-Programming/longest-common-substring.cpp)
+3. [0/1-knapsack](Dynamic-Programming/01-knapsack-bottom-up.cpp)
 
 ## Graphs