Program to build a DFA that checks if a string ends with “01” or “10”

DFA or Deterministic Finite Automata is a finite state machine which accepts a string(under some specific condition) if it reaches a final state, otherwise rejects it.
Problem: Given a string of ‘0’s and ‘1’s character by character, check for the last two characters to be “01” or “10” else reject the string. Also print the state diagram irrespective of acceptance or rejection. Since in DFA, there is no concept of memory, therefore we can only check for one character at a time, beginning with the 0th character. The input set for this problem is {0, 1}. For each character in the input set, each state of DFA redirects to another valid state.
DFA Machine: For the above problem statement, we must first build a DFA machine. DFA machine is similar to a flowchart with various states and transitions. DFA machine corresponding to the above problem is shown below, Q3 and Q4 are the final states: 

Examples:

Input: 010101
Output:
State transitions are q0->q1->q3->q4
->q3->q4->q3->YES

Explanation : 010101 ends with "01".

Input: 0100
Output:
State transitions are q0->q1->q3->q4->q1->NO
Explanation : 0100 ends with "00", 
which is not equal to any of "01" or "10".

Algorithm:

1. Define the minimum number of states required to make the state diagram. Use functions to define various states.
2. List all the valid transitions. Each state must have a transition for every valid symbol.
3. Define the final states by applying the base condition.
4. Define all the state transitions using state function calls.
5. Define a returning condition for the end of the string.

For the given DFA Machine: 

1. Q0, Q1, Q2, Q3, Q4 are defined as the number of states.
2. 0 and 1 are valid symbols. Each state has transitions for 0 and 1.
3. Q3 and Q4 are defined as the final states.
4. Suppose at state Q0, if 0 comes, the function call is made to Q1. So, if 1 comes, the function call is made to Q2.
5. If the program reaches the end of the string, the output is made according to the state, the program is at.

Implementation:

C++

// CPP Program to DFA that accepts string ending
// with 01 or 10.

#include <bits/stdc++.h>
using namespace std;

// Various states of DFA machine are defined
// using functions.
void q1(string, int);
void q2(string, int);
void q3(string, int);
void q4(string, int);

// End position is checked using the string
// length value.
// q0 is the starting state.
// q1 and q2 are intermediate states.
// q3 and q4 are final states.
void q1(string s, int i)
{
    cout << "q1->";

    if (i == s.length()) {
        cout << "NO \n";
        return;
    }

    // state transitions
    // 0 takes to q1, 1 takes to q3
    if (s[i] == '0')
        q1(s, i + 1);
    else
        q3(s, i + 1);
}

void q2(string s, int i)
{
    cout << "q2->";
    if (i == s.length()) {
        cout << "NO \n";
        return;
    }

    // state transitions
    // 0 takes to q4, 1 takes to q2
    if (s[i] == '0')
        q4(s, i + 1);
    else
        q2(s, i + 1);
}

void q3(string s, int i)
{
    cout << "q3->";
    if (i == s.length()) {
        cout << "YES \n";
        return;
    }

    // state transitions
    // 0 takes to q4, 1 takes to q2
    if (s[i] == '0')
        q4(s, i + 1);
    else
        q2(s, i + 1);
}

void q4(string s, int i)
{
    cout << "q4->";
    if (i == s.length()) {
        cout << "YES \n";
        return;
    }

    // state transitions
    // 0 takes to q1, 1 takes to q3
    if (s[i] == '0')
        q1(s, i + 1);
    else
        q3(s, i + 1);
}

void q0(string s, int i)
{
    cout << "q0->";
    if (i == s.length()) {
        cout << "NO \n";
        return;
    }

    // state transitions
    // 0 takes to q1, 1 takes to q2
    if (s[i] == '0')
        q1(s, i + 1);
    else
        q2(s, i + 1);
}

// Driver Code
int main()
{
    string s = "010101";
    // all state transitions are printed.
    // if string is accpetable, YES is printed.
    // else NO is printed
    cout << "State transitions are ";
    q0(s, 0);
}

Java

// Java Program to DFA that accepts string ending
// with 01 or 10.
class GFG
{

    // End position is checked using the string
    // length value.
    // q0 is the starting state.
    // q1 and q2 are intermediate states.
    // q3 and q4 are final states.
    static void q1(String s, int i)
    {
        System.out.print("q1->");
        if (i == s.length())
        {
            System.out.println("NO");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s.charAt(i) == '0')
            q1(s, i + 1);
        else
            q3(s, i + 1);
    }

    static void q2(String s, int i)
    {
        System.out.print("q2->");
        if (i == s.length())
        {
            System.out.println("NO ");
            return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s.charAt(i) == '0')
            q4(s, i + 1);
        else
            q2(s, i + 1);
    }

    static void q3(String s, int i)
    {
        System.out.print("q3->");
        if (i == s.length())
        {
            System.out.println("YES");
            return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s.charAt(i) == '0')
            q4(s, i + 1);
        else
            q2(s, i + 1);
    }

    static void q4(String s, int i)
    {
        System.out.print("q4->");
        if (i == s.length())
        {
            System.out.println("YES");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s.charAt(i) == '0')
            q1(s, i + 1);
        else
            q3(s, i + 1);
    }

    static void q0(String s, int i)
    {
        System.out.print("q0->");
        if (i == s.length())
        {
            System.out.println("NO");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q2
        if (s.charAt(i) == '0')
            q1(s, i + 1);
        else
            q2(s, i + 1);
    }

    // Driver Code
    public static void main (String[] args)
    {
        String s = "010101";

        // all state transitions are printed.
        // if string is accpetable, YES is printed.
        // else NO is printed
        System.out.print("State transitions are ");
        q0(s, 0);
    }
}

// This code is contributed by AnkitRai01

Python3

# Python3 Program to DFA that accepts string ending
# with 01 or 10.

# End position is checked using the string
# length value.
# q0 is the starting state.
# q1 and q2 are intermediate states.
# q3 and q4 are final states.
def q1(s, i) :

    print("q1->", end="");

    if (i == len(s)) :
        print("NO");
        return;

    # state transitions
    # 0 takes to q1, 1 takes to q3
    if (s[i] == '0') :
        q1(s, i + 1);
    else :
        q3(s, i + 1);

def q2(s, i) :

    print("q2->", end = "");
    if (i == len(s)) :
        print("NO");
        return;

    # state transitions
    # 0 takes to q4, 1 takes to q2
    if (s[i] == '0') :
        q4(s, i + 1);
    else :
        q2(s, i + 1);

def q3(s, i) :

    print("q3->", end = "");
    if (i == len(s)) :
        print("YES");
        return;

    # state transitions
    # 0 takes to q4, 1 takes to q2
    if (s[i] == '0') :
        q4(s, i + 1);
    else :
        q2(s, i + 1);

def q4(s, i) :

    print("q4->", end = "");
    if (i == len(s)) :
        print("YES");
        return;

    # state transitions
    # 0 takes to q1, 1 takes to q3
    if (s[i] == '0') :
        q1(s, i + 1);
    else :
        q3(s, i + 1);

def q0( s, i) :

    print("q0->", end = "");
    if (i == len(s)) :
        print("NO");
        return;

    # state transitions
    # 0 takes to q1, 1 takes to q2
    if (s[i] == '0') :
        q1(s, i + 1);
    else :
        q2(s, i + 1);

# Driver Code
if __name__ == "__main__" :
    s = "010101";

    # all state transitions are printed.
    # if string is accpetable, YES is printed.
    # else NO is printed
    print("State transitions are", end = " ");
    q0(s, 0);

# This code is contributed by AnkitRai01

C#

// C# Program to DFA that accepts string ending
// with 01 or 10.
using System;

class GFG
{

    // End position is checked using the string
    // length value.
    // q0 is the starting state.
    // q1 and q2 are intermediate states.
    // q3 and q4 are final states.
    static void q1(string s, int i)
    {
        Console.Write("q1->");
        if (i == s.Length )
        {
            Console.WriteLine("NO");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s[i] == '0')
            q1(s, i + 1);
        else
            q3(s, i + 1);
    }

    static void q2(string s, int i)
    {
        Console.Write("q2->");
        if (i == s.Length)
        {
            Console.WriteLine("NO ");
            return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s[i] == '0')
            q4(s, i + 1);
        else
            q2(s, i + 1);
    }

    static void q3(string s, int i)
    {
        Console.Write("q3->");
        if (i == s.Length)
        {
            Console.WriteLine("YES");
            return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s[i] == '0')
            q4(s, i + 1);
        else
            q2(s, i + 1);
    }

    static void q4(string s, int i)
    {
        Console.Write("q4->");
        if (i == s.Length)
        {
            Console.WriteLine("YES");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s[i] == '0')
            q1(s, i + 1);
        else
            q3(s, i + 1);
    }

    static void q0(string s, int i)
    {
        Console.Write("q0->");
        if (i == s.Length)
        {
            Console.WriteLine("NO");
            return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q2
        if (s[i] == '0')
            q1(s, i + 1);
        else
            q2(s, i + 1);
    }

    // Driver Code
    public static void Main()
    {
        string s = "010101";

        // all state transitions are printed.
        // if string is accpetable, YES is printed.
        // else NO is printed
        Console.Write("State transitions are ");
        q0(s, 0);
    }
}

// This code is contributed by AnkitRai01

Javascript

<script>
      // JavaScript Program to DFA that accepts string ending
      // with 01 or 10.
      // End position is checked using the string
      // length value.
      // q0 is the starting state.
      // q1 and q2 are intermediate states.
      // q3 and q4 are final states.
      function q1(s, i) {
        document.write("q1->");
        if (i === s.length) {
          document.write("NO");
          return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s[i] === "0") q1(s, i + 1);
        else q3(s, i + 1);
      }

      function q2(s, i) {
        document.write("q2->");
        if (i === s.length) {
          document.write("NO ");
          return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s[i] === "0") q4(s, i + 1);
        else q2(s, i + 1);
      }

      function q3(s, i) {
        document.write("q3->");
        if (i === s.length) {
          document.write("YES");
          return;
        }

        // state transitions
        // 0 takes to q4, 1 takes to q2
        if (s[i] === "0") q4(s, i + 1);
        else q2(s, i + 1);
      }

      function q4(s, i) {
        document.write("q4->");
        if (i === s.length) {
          document.write("YES");
          return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q3
        if (s[i] === "0") q1(s, i + 1);
        else q3(s, i + 1);
      }

      function q0(s, i) {
        document.write("q0->");
        if (i === s.length) {
          document.write("NO");
          return;
        }

        // state transitions
        // 0 takes to q1, 1 takes to q2
        if (s[i] === "0") q1(s, i + 1);
        else q2(s, i + 1);
      }

      // Driver Code
      var s = "010101";
      // all state transitions are printed.
      // if string is accpetable, YES is printed.
      // else NO is printed
      document.write("State transitions are ");
      q0(s, 0);
    </script>

Output:

State transitions are q0->q1->q3->q4->q3->q4->q3->YES

Time Complexity: O(n) where a string of length n requires traversal through n states.
Auxiliary Space: O(n)

There can be more than one possible DFA for a problem statement.