I was reading this interesting problem on Stackoverflow:
The problem is not as complex as it seems at first glance. It clearly stipulates a condition for legitimate push: you may only push values in ascending order.
Here, the key is to look at a sequence, and when you see a number k in it, understand that all numbers 0,1,2,3,..,k (all m < k) get pushed first. That is, you may not push any r > k before k has itself been pushed on to the stack.
With the retention of that simple idea, you can land at the solution pretty easily.
