In the problem description its mention that we cant use extra space. So does that mean we cant use integer variable for storing previous value or we can use it?
Hey I suggest you learn about space complexity and time complexity.
Space Complexity : How your space increases as your input to the program becomes exponential and similar definition would work for time as well.
Constant space or no extra space : O(1)
In a nutshell : you are allowed to use as many variables as you want to solve problem.
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