PROBLEM LINKS
DIFFICULTY
EASY
EXPLANATION
This problem can be solved by dynamic programming. Let dp[i][j] represent number of ways of arrangement such that in a stack of i pancakes the maximum radius is j. hence dp[i][j] = dp[i-1][j]*j + dp[i-1][j-1]. The first term is the cases in which you have placed pancakes till size j in the previous position, in those cases you can place any pancake till radius j in the current position. And the second term is the case when you are placing a pancake of radius exactly 1 less than j in all the previous positions and you are allowed to place a pancake of radius j in the current position. For answer of size n, sum over all j <= n for dp[n][j] will be the result.
SETTER’S SOLUTION
Can be found here.
TESTER’S SOLUTION
Can be found here.