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Question 1: Consider the following sequence of push operations in a stack:
stack.push('1');
stack.push('2');
stack.push('3');
stack.push('4');
stack.push('5');
stack.push('6');
You can insert as many stack.pop()'s as you like in the above sequence of stack.push's to get a desired output. Which of the following cannot be an output?

1. 123456
2. 325416
3. 342561
4. 342615
5. E. 342165