Dynamic Programming
Dynamic programming refers to a problem solving approach, in which we precompute and store simpler, similar subproblems, in order to build up the solution to a complex problem. It is similar to recursion, in which calculating the base cases allow us to inductively determine the final value. This bottomup approach works well when the new value depends only on previously calculated values.
An important property of a problem that is being solved through dynamic programming is that it should have overlapping subproblems. This is what distinguishes DP from Divide and Conquer in which storing the simpler values aren't necessary.
To show how powerful the technique can be, here are some of the most famous problems commonly approached through Dynamic Programming:
 Backpack Problem Given a set of treasures with known values and weights, which of them should you pick to maximize your profit whilst not damaging your backpack which has a fixed capacity?
 Egg Dropping What is the best way to drop n eggs from an m floored building to figure out the lowest height from which the eggs when dropped cracks?
 Longest Common Subsequence Given two sequences, which is the longest subsequence common to both of them.
 Subset Sum Problem Given a set and a value n, is there a subset the sum of whose elements is n?
 Fibonacci Numbers Is there a better way to compute fibonacci numbers than plain recursion?
In a contest environment, dynamic programming almost always comes up (and often in a surprising way, no matter how familiar the contestant is with it).
Contents
Motivational Example: Change of Coins
Problem: What is the minimum number of coins of values \(v_1,v_2, v_3, \cdots v_n\) required to amount a total of \(V\)?
You may use a denomination more than once.
Optimal Substructure
The most important aspect of this problem that encourages us to solve this through dynamic programming is that it can be simplified to smaller subproblems.
Let \(f(N)\) represent the minimum number of coins required for a value of \(N\).
Visualise \(f(N)\) as a stack of coins. What is the coin at the top of the stack? It could be any of \(v_1,v_2, v_3, \cdots v_n\). In case it were \(v_1\), the rest of the stack would amount to \(Nv_1\) or if it were \(v_2\), the rest of the stack would amount to \(Nv_2\), and so on.
How do we decide which is it? Sure enough, we do not know yet. We need to see which of them minimizes the number of coins required.
Going by the above argument, we could state the problem as follows:
\[f(V) = \min \left ( \left \{ 1 + f(V  v_1), 1 + f(Vv_2), \cdots, 1 + f(Vv_n) \right \} \right ) \]
Because the coin at the top of the stack also counts as one coin, and then we can look at the rest.
Overlapping Subproblems
It is easy to see that the subproblems could be overlapping.
For example, if we are trying to make a stack of $11 using $1, $2 and $5, our look up pattern would be like this:
\[ f(11) = \min \left ( \left \{ 1+f(10), 1+ f(9), 1 + f(6) \right \} \right ) \\ = \min \left ( \left \{ 1+ \min \left ( \left \{ 1 + f(9), 1+ f(8), 1+ f(5) \right \} \right ), 1+ f(9), 1 + f(6) \right \} \right ) \]
Clearly enough, we'll need to use the value of \(f(9)\) several times
One of the most important aspects of optimising our algorithms is that we do not recompute these values. To do this, we compute and store all the values of \(f\) from 1 onwards for potential future use.
Edge Cases
The recursion has to bottom out somewhere, in other words, a known value from which it can start.
For this problem, we need to take care of two things:
Zero: It is clear enough that \(f(0) = 0\) since we do not require any coins at all to make a stack amounting to 0.
Negative and Unreachable Values: One way of dealing with such values is to mark them with a sentinel value so that our code deals with them in a special way. A good choice of a sentinel is \(\infty\), since the minimum value between a reachable value and \(\infty\) could never be the reachable value.
The Algorithm
Let's sum up the ideas and see how could implement this as an actual algorithm:
1 2 3 4 5 6 7 8 

Recursion with Memoization
We have claimed that naive recursion is a bad way to solve problems with overlapping subproblems. Why is that? Mainly because of all the recomputations involved.
Another way to avoid this problem is to compute the data first time and store it as we go, in a top down fashion.
Let's look at how one could potentially solve the previous coin change problem in the memoization way.
1 2 3 4 5 6 7 8 9 10 11def coinsChange(V,v): memo = {} def Change(V): if V in memo: return memo[V] if V == 0: return 0 if V < 0: return float("inf") return min([1+Change(Vvi) for vi in v]) return Change(V)
Dynamic Programming vs Recursion with Caching
Dynamic Programming  Recursion with Caching 
Intuitive approach  Faster if many subproblems are visited as there is no overhead from recursive calls 
Computes only those subproblems which are necessary  The complexity of the program is easier to see 
Bidimensional Dynamic Programming: Example
There are \(k\) types of brackets each with its own opening bracket and closing bracket. We assume that the first pair is denoted by the numbers 1 and \(k+1\), the second by 2 and \(k+2\) and so on. Thus the opening brackets are denoted by \(1, 2, \ldots , k\), and the corresponding closing brackets are denoted by \(k+1, k+2, \ldots, 2k\) respectively.
Some sequences with elements from \(1, 2, \ldots, 2k\) form well bracketed sequences while others don't. A sequence is well bracketed, if we can match or pair up opening brackets of the same type in such a way that the following holds:
 Every bracket is paired up.
 In each matched pair, the opening bracket occurs before the closing bracket.
 For a matched pair, any other matched pair lies either completely between them or outside them.
In this problem you are given a sequence of brackets, of length \(N\): \(B[1], \ldots , B[N]\), where each \(B[i]\) is one of the brackets. You are also given an array of Values: \(V[1],\ldots, V[N] \).
Among all the subsequences in the Values array, such that the corresponding bracket subsequence in the B Array is a wellbracketed sequence, you need to find the maximum sum.
Task: Solve the above problem for this input.
Input Format
One line, which contains (2*N + 2) space separate integers. The first integer denotes N. The next integer is k. The next N integers are V[1],..., V[N]. The last N integers are B[1],.., B[N].
Constraints
 \(1 \leq k \leq 7\)
 \(10^6 \leq V[i] \leq 10^6\), for all \(i\)
 \(1 \leq B[i] \leq 2k\), for all \(i\)
Illustrated Examples
 For the examples discussed here, let us assume that \(k = 2\). The sequence 1,1,3 is not wellbracketed as one of the two 1's cannot be paired. The sequence 3,1,3,1 is not wellbracketed as there is no way to match the second 1 to a closing bracket occurring after it. The sequence 1,2,3,4 is not wellbracketed as the matched pair 2,4 is neither completely between the matched pair 1,3 nor completely outside it. That is, the matched pairs cannot overlap. The sequence 1,2,4,3,1,3 is wellbracketed. We match the first 1 with the first 3, the 2 with the 4 and the second 1 with the second 3, satisfying all the 3 conditions. If you rewrite these sequences using [,{,],} instead of 1,2,3,4 respectively, this will be quite clear.
*Suppose N = 6, k = 3 and the values of V and B are as follows:
Then, the brackets in positions 1,3 form a wellbracketed sequence (1,4) and the sum of the values in these positions is 2 (4 + 2 =2). The brackets in positions 1,3,4,5 form a wellbracketed sequence (1,4,2,5) and the sum of the values in these positions is 4. Finally, the brackets in positions 2,4,5,6 forms a wellbracketed sequence (3,2,5,6) and the sum of the values in these positions is 13. The sum of the values in positions 1,2,5,6 is 16 but the brackets in these positions (1,3,5,6) do not form a wellbracketed sequence. You can check the best sum from positions whose brackets form a wellbracketed sequence is 13.
We'll try to solve this problem with the help of a dynamic program, in which the state, or the parameters that describe the problem, consist of two variables.
First, we setup a two dimensional array dp[start][end]
where each entry solves the indicated problem for the part of the sequence between start
and end
inclusive.
We'll try to think what happens when we run across a new end
value, and need to solve the new problem in terms of the previously solved subproblems. Here are all the possibilites:
 When
end <= start
, there are no valid subsequences.  When
b[end] <= k
, i.e, the last entry is an open bracket, no valid subsequence can end with it. Effectively, the result is the same if we hadn't included the last entry at all.  When
b[end] > k
, i.e, the last entry is a closing bracket, one has to find the best match for it, or simply ignore it, whichever maximizes the sum.
Can you use this ideas to solve the problem?
Example: Maximum Paths
Very often, dynamic programming helps solve problems that ask us to find the most profitable (or least cost) path in an implicit graph setting. Let us try to illustrate this with an example.
You are supposed to start at the top of a number triangle and chose your passage all the way down by selecting between the numbers below you to the immediate left or right.
Your goal is to maximise the sum of the elements lying in your path.
For example, in the triangle below, the red path maximizes the sum.
To see the optimal substructures and the overlapping subproblems, notice that everytime we make a move from the top to the bottom right or the bottom left, we are still left with smaller number triangle, much like this:
We could break each of the subproblems in a similar way until we reach an edgecase at the bottom:
In which case, the solution isa + max(b,c)
A bottom up dynamic programming solution is to allocate a number triangle that stores the maximum reachable sum if we were to start from that position. It is easy to compute the number triangle from the bottom row onward using the fact that the \[\text{best from this point} = \text{this point} + \max(\text{best from the left, best from the right})\]
Let me demonstrate this principle through the iterations.
Iteration 1:
18 5 9 3
Iteration 2:
1 210 13 15 8 5 9 3
Iteration 3:
1 2 320 19 10 13 15 8 5 9 3
Iteration 4:
1 2 3 423 20 19 10 13 15 8 5 9 3
So, the effective best we could do from the top is 23, which is our answer.