Check if two strings have a common substring

Last Updated : 27 Jan, 2023

You are given two strings str1 and str2. You have to check if the two strings share a common substring.

Examples : 

Input : str1 = "HELLO"
        str2 = "WORLD"
Output : YES
Explanation :  The substrings "O" and
"L" are common to both str1 and str2

Input : str1 = "HI"
        str2 = "ALL"
Output : NO
Explanation : Because str1 and str2 
have no common substrings

A basic approach runs in O(n^2), where we compare every character of string 1 with every character of string 2 and replace every matched character with a "_" and set flag variable as true.

An efficient approach works in O(n). We basically need to check if there is a common character or not. We create a vector of size 26 for alphabets and initialize them as 0. For every character in string 1 we increment vector index of that character eg: v[s1[i]-'a']++, for every character of string 2 we check vector for the common characters if v[s2[i]-'a'] > 0 then set flag = true and v[s2[i]-'a']-- such that one character of string 2 is compared with only one character of string 1.

Implementation:

C++ 
// CPP program to check if two strings have
// common substring
#include <bits/stdc++.h>
using namespace std;

const int MAX_CHAR = 26;

// function to return true if strings have 
// common substring and no if strings have
// no common substring
bool twoStrings(string s1, string s2) {

  // vector for storing character occurrences
  vector<bool> v(MAX_CHAR, 0);

  // increment vector index for every
  // character of str1
  for (int i = 0; i < s1.length(); i++)
    v[s1[i] - 'a'] = true;

  // checking common substring of str2 in str1
  for (int i = 0; i < s2.length(); i++) 
    if (v[s2[i] - 'a']) 
       return true;
 
  return false;        
}

// driver program
int main() {
  string str1 = "hello";
  string str2 = "world";
  if (twoStrings(str1, str2))
     cout << "Yes";
  else
     cout << "No";
  return 0;
}
Java 
// Java program to check if two strings have
// common substring
import java.util.Arrays;

class GFG
{
    static int MAX_CHAR = 26;
    
    // function to return true if strings have 
    // common substring and no if strings have
    // no common substring
    static boolean twoStrings(String s1, String s2) 
    {
        // vector for storing character occurrences
        boolean v[]=new boolean[MAX_CHAR];
        Arrays.fill(v,false);
    
        // increment vector index for every
        // character of str1
        for (int i = 0; i < s1.length(); i++)
            v[s1.charAt(i) - 'a'] = true;
        
        // checking common substring of str2 in str1
        for (int i = 0; i < s2.length(); i++) 
            if (v[s2.charAt(i) - 'a']) 
            return true;
        
        return false;     
    }
    
    // Driver code
    public static void main (String[] args) 
    {
        String str1 = "hello";
        String str2 = "world";
        if (twoStrings(str1, str2))
            System.out.print("Yes");
        else
            System.out.print("No");
    }
}

// This code is contributed by Anant Agarwal.
Python3 
# Python3 code to implement the approach

# python program to check if two strings have
# common substring
MAX_CHAR = 26

# function to return true if strings have 
# common substring and no if strings have
# no common substring
def twoStrings(s1, s2):

    # vector for storing character occurrences
    v = [0 for i in range(MAX_CHAR)]

    # increment vector index for every
    # character of str1
    for i in range(len(s1)):
        v[ord(s1[i]) - ord('a')] = True

    # checking common substring of str2 in str1
    for i in range(len(s2)):
        if v[ord(s2[i]) - ord('a')]:
            return True
    return False

# driver program
str1 = "hello"
str2 = "world"
if twoStrings(str1, str2):
    print("Yes")
else:
    print("No")

# This code is contributed by phasing17
C# 
// C# program to check if two 
// strings have common substring
using System;

class GFG
{
    static int MAX_CHAR = 26;
    
    // function to return true if strings have 
    // common substring and no if strings have
    // no common substring
    static bool twoStrings(String s1, String s2) 
    {
        // vector for storing character occurrences
        bool []v = new bool[MAX_CHAR];
        
    // Arrays.fill(v,false);
    for(int i = 0; i < MAX_CHAR; i++)
    v[i]=false;
    
        // increment vector index for 
        // every character of str1
        for (int i = 0; i < s1.Length; i++)
            v[s1[i] - 'a'] = true;
        
        // checking common substring of str2 in str1
        for (int i = 0; i < s2.Length; i++) 
            if (v[s2[i] - 'a']) 
            return true;
        
        return false;     
    }
    
    // Driver code
    public static void Main () 
    {
        String str1 = "hello";
        String str2 = "world";
        if (twoStrings(str1, str2))
            Console.Write("Yes");
        else
            Console.Write("No");
    }
}

// This code is contributed by nitin mittal.
JavaScript 
<script>

// javascript program to check if two strings have
// common substring

var MAX_CHAR = 26;

// function to return true if strings have 
// common substring and no if strings have
// no common substring
function twoStrings(s1, s2) {

  // vector for storing character occurrences
  var v = Array(MAX_CHAR).fill(0);

  // increment vector index for every
  // character of str1
  for (var i = 0; i < s1.length; i++)
    v[s1[i] - 'a'] = true;

  // checking common substring of str2 in str1
  for (var i = 0; i < s2.length; i++) 
    if (v[s2[i] - 'a']) 
       return true;
 
  return false;        
}

// driver program
var str1 = "hello";
var str2 = "world";
if (twoStrings(str1, str2))
   document.write( "Yes");
else
   document.write("No");

// This code is contributed by rutvik_56.
</script>

Output
Yes

Time Complexity : O(n) 
Auxiliary Space: O(1)

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