Alice Zhao

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Today I Learned (TIL): Dynamic Programming

There are 2 words that send shiver down a programmer's spine: Dynamic Programming.

Did you know the name was picked by researcher Dr. Richard Bellman to sound cool so he can get funding?

Here is what I learned today about DP!

Code sniff

  1. Optimization problems: shortest/longest, minimized/maximized, least/most, etc
  2. Can the problem can be broken down into a collection of simpler subproblems?
  • Divide and Conquer approach: Divide problem into a small number of disjoint subproblems
  • DP: Divide problem into many potential overlapping subproblems and storing the solution to save computation time.

Example

There are n step staircase. To climb it, you can take either one or two steps at a time. How many different ways is it possible to reach the top?

Strategy

Step 1: Identify problem variables.

To achieve the goal, what are all the options to consider at each step? What parameters are changing at each subproblem?

  • Staircase P: 1 changing parameter: n: number of stairs. Option to take 1 step or 2 steps.

Step 2: Identify subproblems.

Typical scenarios are:

  • If problem involves array/ matrices, subproblems would be array of smaller sizes: A[0 ... i-1] as it relates to A[0 ... i]
  • If a problem involves a discrete quantity[i.e number of items, amt of money rounded to dollar, weight rounded to kg]: subproblems for 5 item set: look for solutions for sets of 4, 3, 2, 1, 0 items
  • Staircase P: The number of steps is a discrete quantity. A smaller problem is knowing the number of different ways to go to top of n - 1 and n - 2 staircase.

Step 3: Identify base cases

A base case is a subproblem that doesn't depend on other subproblems: hit a constraint or edge case.

  • Staircase P: When the stairs given is 1 or 2

Step 4: Determine how subproblem relate to the main problem: Recurrence relation.

  • Based on step 2 and 3 and assuming you have computed the subproblems, how would you compute the main problem?
  • Staircase P:
    • Stairs[1] = 1
    • Stairs[2] = 2
    • Stairs[n] = Stairs[n - 2] + Stairs[n - 1]

Step 5: Determine time complexity

  • Count the number of states: this depends on number of changing parameters
  • Think about work done at each state. If everything else but one state has been computed, how much work do you compute that last state?
  • Staircase P: States: n Work: O(1)

How can we implement?

Approach 1: Recursion (Top down)

Approach 2: Add memoization.

This can greatly speed up our recursion. It stores the results of expensive function calls and return the cached result. 1. Initiate an object to act as memo: add optional parameter 2. Store function result into memory before every return statement 3. Look up the memory for the function result before you start doing any other computation.

Approach 3: Dynamic Programming (Bottom Up)

Approach 4: Pointers/ Iterative: If recurrence is only based on previous few subproblems, you can use pointers to store these variables. Else you have to keep track with an entire array.

Check out other algo patterns I've learned!

Check out to see more examples of strategies to solve DP problems!