Find the longest sub-string which is prefix, suffix and also present inside the string | Set 2

Given string str. The task is to find the longest sub-string which is a prefix, a suffix and a sub-string of the given string, str. If no such string exists then print -1.

Examples:

Input: str = “geeksisforgeeksinplatformgeeks”
Output: geeks



Input: str = “fixprefixsuffix”
Output: fix

Note: The Set-1 of this article is attached here.

Approach:
The implementation is using BIT and Z algorithm. Learn more about them from this and this.

  • First we are calculating the Z array by using the Z algorithm.
  • Update the values in Bit array by 1 from index z[i].
  • Querying for the maximum length needed substring using the pref function.
  • If len is 0 then such substring is not possible from the given string.

Below is the implementation approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ program to find that
// substring which is its
// suffix prefix and also
// found somewhere in betweem
#include <bits/stdc++.h>
using namespace std;
  
// Z-algorithm function
vector<int> z_function(string s)
{
    int n = s.size();
    vector<int> z(n);
    for (int i = 1, l = 0,
             r = 0;
         i < n;
         i++) {
        if (i <= r)
            z[i] = min(r - i + 1,
                       z[i - l]);
  
        while (i + z[i] < n
               && s[z[i]] == s[i + z[i]])
            z[i]++;
  
        if (i + z[i] - 1 > r)
            l = i, r = i + z[i] - 1;
    }
    return z;
}
  
int n, len = 0;
  
// BIT array
int bit[1000005];
  
string s;
vector<int> z;
map<int, int> m;
  
// bit update function which
// updates values from index
// "idx" to last by value "val"
void update(int idx, int val)
{
    if (idx == 0)
        return;
    while (idx <= n) {
        bit[idx] += val;
        idx += (idx & -idx);
    }
}
  
// Query function in bit
int pref(int idx)
{
    int ans = 0;
    while (idx > 0) {
        ans += bit[idx];
        idx -= (idx & -idx);
    }
    return ans;
}
  
// Driver Code
int main()
{
    s = "geeksisforgeeksinplatformgeeks";
    n = s.size();
  
    // Making the z array
    z = z_function(s);
  
    // update in the bit array from
    // index z[i] by increment of 1
    for (int i = 1; i < n; i++) {
        update(z[i], 1);
    }
  
    for (int i = n - 1; i > 1; i--) {
        // if the value in z[i] is
        // not equal to (n-i) then no
        // need to move further
        if (z[i] != (n - i))
            continue;
  
        // queryng for the maximum
        // length substring from
        // bit array
        if (pref(n) - pref(z[i] - 1) >= 2) {
            len = max(len, z[i]);
        }
    }
  
    if (!len)
        cout << "-1";
    else
        cout << s.substr(0, len);
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java program to find that
// substring which is its
// suffix prefix and also
// found somewhere in betweem
class GFG
{
  
// Z-algorithm function
static int[] z_function(char []s)
{
    int n = s.length;
    int []z = new int[n];
    for (int i = 1, l = 0, r = 0
             i < n; i++) 
    {
        if (i <= r)
            z[i] = Math.min(r - i + 1,
                            z[i - l]);
  
        while (i + z[i] < n &&
                 s[z[i]] == s[i + z[i]])
            z[i]++;
  
        if (i + z[i] - 1 > r)
        {
            l = i; r = i + z[i] - 1;
        }
    }
    return z;
}
  
static int n, len = 0;
  
// BIT array
static int []bit = new int[1000005];
  
static String s;
static int[] z;
  
// bit update function which
// updates values from index
// "idx" to last by value "val"
static void update(int idx, int val)
{
    if (idx == 0)
        return;
    while (idx <= n)
    {
        bit[idx] += val;
        idx += (idx & -idx);
    }
}
  
// Query function in bit
static int pref(int idx)
{
    int ans = 0;
    while (idx > 0)
    {
        ans += bit[idx];
        idx -= (idx & -idx);
    }
    return ans;
}
  
// Driver Code
public static void main(String[] args)
{
    s = "geeksisforgeeksinplatformgeeks";
    z = new int[s.length()];
    n = s.length();
  
    // Making the z array
    z = z_function(s.toCharArray());
  
    // update in the bit array from
    // index z[i] by increment of 1
    for (int i = 1; i < n; i++) 
    {
        update(z[i], 1);
    }
  
    for (int i = n - 1; i > 1; i--)
    {
        // if the value in z[i] is
        // not equal to (n-i) then no
        // need to move further
        if (z[i] != (n - i))
            continue;
  
        // queryng for the maximum
        // length substring from
        // bit array
        if (pref(n) - pref(z[i] - 1) >= 2
        {
            len = Math.max(len, z[i]);
        }
    }
  
    if (len == 0)
        System.out.println("-1");
    else
        System.out.println(s.substring(0, len));
}
}
  
// This code is contributed 
// by PrinciRaj1992

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 program to find that
# substring which is its
# suffix prefix and also
# found somewhere in betweem
  
# Z-algorithm function
def z_function(s):
    global z, n
    n = len(s)
    z = [0] * n
    l, r = 0, 0
    for i in range(1, n):
        if i <= r:
            z[i] = min(r - i + 1, z[i - 1])
  
        while (i + z[i] < n and s[z[i]] == s[i + z[i]]):
            z[i] += 1
  
        if (i + z[i] - 1 > r):
            l = i
            r = i + z[i] - 1
    return z
  
# bit update function which
# updates values from index
# "idx" to last by value "val"
def update(idx, val):
    global bit
    if idx == 0:
        return
    while idx <= n:
        bit[idx] += val
        idx += (idx & -idx)
  
# Query function in bit
def pref(idx):
    global bit
    ans = 0
    while idx > 0:
        ans += bit[idx]
        idx -= (idx & -idx)
  
    return ans
  
# Driver Code
if __name__ == "__main__":
    n = 0
    length = 0
  
    # BIT array
    bit = [0] * 1000005
    z = []
    m = dict()
  
    s = "geeksisforgeeksinplatformgeeks"
  
    # Making the z array
    z = z_function(s)
  
    # update in the bit array from
    # index z[i] by increment of 1
    for i in range(1, n):
        update(z[i], 1)
  
    for i in range(n - 1, 1, -1):
  
        # if the value in z[i] is
        # not equal to (n-i) then no
        # need to move further
        if z[i] != n - i:
            continue
  
        # queryng for the maximum
        # length substring from
        # bit array
        if (pref(n) - pref(z[i] - 1)) >= 2:
            length = max(length, z[i])
  
    if not length:
        print(-1)
    else:
        print(s[:length])
  
# This code is contributed by
# sanjeev2552

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program to find that
// substring which is its
// suffix prefix and also
// found somewhere in betweem
using System;
  
class GFG
{
  
// Z-algorithm function
static int[] z_function(char []s)
{
    int n = s.Length;
    int []z = new int[n];
    for (int i = 1, l = 0, r = 0; 
             i < n; i++) 
    {
        if (i <= r)
            z[i] = Math.Min(r - i + 1,
                            z[i - l]);
  
        while (i + z[i] < n &&
                 s[z[i]] == s[i + z[i]])
            z[i]++;
  
        if (i + z[i] - 1 > r)
        {
            l = i; r = i + z[i] - 1;
        }
    }
    return z;
}
  
static int n, len = 0;
  
// BIT array
static int []bit = new int[1000005];
static String s;
static int[] z;
  
// bit update function which
// updates values from index
// "idx" to last by value "val"
static void update(int idx, int val)
{
    if (idx == 0)
        return;
    while (idx <= n)
    {
        bit[idx] += val;
        idx += (idx & -idx);
    }
}
  
// Query function in bit
static int pref(int idx)
{
    int ans = 0;
    while (idx > 0)
    {
        ans += bit[idx];
        idx -= (idx & -idx);
    }
    return ans;
}
  
// Driver Code
public static void Main(String[] args)
{
    s = "geeksisforgeeksinplatformgeeks";
    z = new int[s.Length];
    n = s.Length;
  
    // Making the z array
    z = z_function(s.ToCharArray());
  
    // update in the bit array from
    // index z[i] by increment of 1
    for (int i = 1; i < n; i++) 
    {
        update(z[i], 1);
    }
  
    for (int i = n - 1; i > 1; i--)
    {
        // if the value in z[i] is
        // not equal to (n-i) then no
        // need to move further
        if (z[i] != (n - i))
            continue;
  
        // queryng for the maximum
        // length substring from
        // bit array
        if (pref(n) - pref(z[i] - 1) >= 2) 
        {
            len = Math.Max(len, z[i]);
        }
    }
  
    if (len == 0)
        Console.WriteLine("-1");
    else
        Console.WriteLine(s.Substring(0, len));
}
}
  
// This code is contributed by PrinciRaj1992

chevron_right


Output:

geeks

Time complexity: O(N)



My Personal Notes arrow_drop_up

Competitive Programmer, Full Stack Developer, Technical Content Writer, Machine Learner

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : princiraj1992, sanjeev2552



Article Tags :
Practice Tags :


Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.