Answer:
For the given input string "abcd", the permutation with an output 'd' is given by "dcba".
Explanation:
This is because for a output strings to begin with 'd', we first need to input and push characters 'a','b','c' and 'd' to stack
cin >> ch; permuteStack.push(ch); // inputs and push 'a' into the stack
cin >> ch; permuteStack.push(ch); // inputs and push 'b' into the stack
cin >> ch; permuteStack.push(ch); // inputs and push 'c' into the stack
cin >> ch; permuteStack.push(ch); // inputs and push 'd' into the stack
Then we need to pop and display character 'd' from stack to console
ch = permuteStack.pop(); cout << ch; // pops and outputs 'd' to console
After popping 'd' from stack , we can only pop the characters in the order 'c','b' and 'a'. No other popping of characters can be done, because, we have taken all the inputs and only operation which can be performed is popping and displaying.
ch = permuteStack.pop(); cout << ch; // pops and outputs 'c' to console
ch = permuteStack.pop(); cout << ch; // pops and outputs 'b' to console
ch = permuteStack.pop(); cout << ch; // pops and outputs 'a' to console
So the only permutation which can be displayed starting with 'd' is "dcba"
