implement DP for a recursive function

Question

I have the following recursive function:

typedef unsigned long long ull;

ull calc(ull b, ull e)
{
  if (!b) return e;
  if (!e) return b;
  return calc(b - 1, e - 1) + calc(b - 1, e) - calc(b, e - 1);
}

I want to implement it with dynamic programming (ie using storage). I have tried to use a map<pair<ull, ull>, ull> but it is too slow also. I couldn't implement it using arrays O(1) too.

I want to find a solution so that this function solves quickly for large b, e s.

Answer 1

Make a table b/e and fill it cell by cell. This is DP with space and time complexity O(MaxB*MaxE).

Space complexity may be reduced with Ante's proposal in comment - store only two needed rows or columns.

0 1 2 3 4 5
1 0 3 . . .
2 . . . . .
3 . . . . .
4 . . . . .

Answer 2

You might want to take a look at this recent blog posting on general purpose automatic memoization . The author discusses various data structures, such std::map , std::unordered_map etc. Warning: uses template-heavy code.

Answer 3

If a bottom up representation is what you want then this would do fine.

Fill up the table as MBo has shown

This can be done as:

for e from 0 to n:
  DP[0][e] = e
for b from 0 to n:
  DP[b][0] = b
for i from 1 to n:
   for j from 1 to n:
      DP[i][j] = DP[i-1][j-1] + DP[i-1][j] - DP[i][j-1]

now your answer for any b,e is simply DP[b][e]

Answer 4

You can implement in O(n^2) (assuming n as max number of values for b and e ) by using a 2 dimensional array. Each current value for i,j would depend on the value at i-1,j and i-1,j-1 and i,j-1. Make sure you handle cases for i=0, j=0.