Longest string which is prefix string of at least two strings


Given a set of strings of the same length, we need to find the length of the longest string which is a prefix string of at least two strings.

Examples:

Input:  ["abcde", "abcsd", "bcsdf", "abcda", "abced"]
Output: 4
Explanation:  
Longest prefix string is "abcd".

Input:  ["pqrstq", "pwxyza", "abcdef", "pqrstu"]
Output: 5

Approach:

  • Starting from 0th position iterate over every character and check if that character occurs in at least two of the string at the current position or not.
  • If occurs, then recursively call on next position. Otherwise,
  • Update the max value by taking maximum with Current_position – 1.
  • At last, return the max value.

C++

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// C++ program to find longest
// string which is prefix string
// of at least two strings
#include<bits/stdc++.h>
using namespace std;
int max1=0;
  
// Function to find Max length 
// of the prefix
int MaxLength(vector<string> v, int i, 
                                int m)
{
    // Base case
    if(i>=m)
    {
        return m-1;
    }
      
    // Iterating over all the alphabets
    for(int k = 0; k < 26; k++)
    {
        char c = 'a' + k;
        vector<string> v1;
          
        // Checking if char exists in
        // current string or not
        for(int j = 0; j < v.size(); j++)
        {
            if(v[j][i] == c)
            {
                v1.push_back(v[j]);
            }
        }
          
        // If atleast 2 string have 
        // that character
        if(v1.size()>=2)
        {
           // Recursive call to i+1 
           max1=max(max1,
                    MaxLength(v1, i+1, m));
        }
        else
        {
            max1=max(max1, i - 1);
        }
    }
    return max1;
}
  
// Driver code
int main()
{
  // Initialising strings  
  string s1, s2, s3, s4, s5;
    
  s1 = "abcde";
  s2 = "abcsd";
  s3 = "bcsdf";
  s4 = "abcda";
  s5 = "abced";
       
  vector<string> v;
      
  // push strings into vectors.
  v.push_back(s1);
  v.push_back(s2);
  v.push_back(s3);
  v.push_back(s4);
  v.push_back(s5);
      
  int m = v[0].size();
      
  cout<<MaxLength(v, 0, m) + 1<<endl;
   
  return 0;

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Python3

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# Python3 program to find longest
# string which is prefix string
# of at least two strings
  
max1 = 0
  
# Function to find max length 
# of the prefix
def MaxLength(v, i, m):
      
    global max1
      
    # Base case
    if(i >= m):
        return m - 1
          
    # Iterating over all the alphabets
    for k in range(26):
        c = chr(ord('a') + k)
        v1 = []
          
        # Checking if char exists in
        # current string or not
        for j in range(len(v)):
            if(v[j][i] == c):
                v1.append(v[j])
          
        # If atleast 2 string have 
        # that character
        if(len(v1) >= 2):
              
            # Recursive call to i+1
            max1 = max(max1, MaxLength(v1, i + 1, m))
        else:
            max1 = max(max1, i - 1)
              
    return max1
  
# Driver code
if __name__ == '__main__':
      
    # Initialising strings 
    s1 = "abcde"
    s2 = "abcsd"
    s3 = "bcsdf"
    s4 = "abcda"
    s5 = "abced"
    v = []
  
    # Push strings into vectors.
    v.append(s1)
    v.append(s2)
    v.append(s3)
    v.append(s4)
    v.append(s5)
      
    m = len(v[0])
      
    print(MaxLength(v, 0, m) + 1)
  
# This code is contributed by BhupendraSingh

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Output:

4

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