# Construct Pushdown automata for L = {0m1(n+m)2n | m,n ≥ 0}

• Difficulty Level : Medium
• Last Updated : 11 Feb, 2019

Prerequisite – Pushdown automata, NPDA for accepting the language L = {amb(n+m)cm | m, n >= 1}
Problem: Construct Pushdown automata for L = {0m1(n+m)2n | m,n ≥ 0}

Example:

Input: 011122
Output: Accepted

Input: 00000112222
Output: Not Accepted

Approach used in this PDA –
There can be four cases while processing the given input string.

Case 1- m=0:
In this cases the input string will be of the form {1n2n}. In this condition, keep on pushing 1’s in the stack until we encounter with 2. On receiving 2 check if top of stack is 1, then pop it from the stack. Keep on popping 1’s until all the 2’s of the string are processed. If we reach to the end of input string and stack becomes empty, then reached to the final state i.e. Accepts the input string else move to dead state.

Case 2- n=0:
In this cases the input string will be of the form {0m1m}. In this condition, keep on pushing 0’s in the stack until we encounter with 1. On receiving 1 check if top of stack is 0, then pop it from the stack. Keep on popping 0’s until all the 1’s of the string are processed. If we reach to the end of input string and stack becomes empty, then reached to the final state i.e. Accepts the input string else move to dead state.

Case 3- m, n>0:
In this cases the input string will be of the form {0m1(m+n)2n}. In this condition, keep on pushing 0’s in the stack until we encounter with 1. On receiving 1 check if top of stack is 0 then pop it from the stack. Keep on popping 0’s until all the 0’s are popped and stack became empty.After that, keep on pushing 1’s in the stack until we encounter with 2.

On receiving 2 check if top of stack is 1 then pop it from the stack. Keep on popping 1’s until all the 2’s of the input string are processed If we reach to the end of input string and stack becomes empty, then reach to final state i.e. accept the input string else move to dead state.

Case 4- m=0, n=0:
In this case the input string will be empty. Therefore directly jump to final state.

State transition diagram:

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