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Introduction To Dynamic ProgrammingDP is a very useful and effective technique for finding optimal solution for problems having exponential time complexities( O(n!) or O(2^n) )
as it may bring down the complexity to O(n^2) or O(n^3).DP applies to the problems having overlapping sub-problems.DP computes sub-problems before the problem itself and combines value of sub-problem to generate solution to the problem.
Since problems are overlapping we may end up computing same sub-problems over and over again.So, DP speeds up the process by storing those results in a table and fetch result instead of computing it again.For ex :
Take an example of Fibonacci series, fibo(4) and fibo(3) are sub-problems to fibo(5) and as we can see from image we’ll compute fibo(3) twice one while computing fibo(5) and one while computing fibo(4), that’s a lot of wastage, in DP technique we’ll store fibo(3) and use this result when required.Let’s take a simple example:
Problem: Given an array of integers of size n.We’ve to answer Q queries.Each query consists of 2 integers L and R
and we’ve to print sum of elements of array from L to R(inclusive).input:
5
1 2 3 4 5
2
1 5
2 5
output:
15
14Naive: For each query iterate through L to R and calculate the sumfor q<-1 upto Q:
sum = 0
for i<-L upto R:
sum = sum + Arr[i]
print sum.:for single Query takes O(n) time and for answering Q queries it takes O(Q*n) time.DP : Find and store the cumulative frequency in a table(array)table[1…n] = {0}
table[0] = Arr[0]
for i<-1 upto n:
table[i] = table[i-1] + Arr[i]this pre-computation requires O(n) time.
Now we can answer each query in O(1) time as follows:for q<-1 upto Q:
print (table[R] – table[L-1]).: for Q queries complexity becomes O(1*Q) = O(Q)So, overall complexity of problem using DP is: O(Q+n) which is linear in time.
Though DP requires Extra space of order O(n), but the reduction in the time complexity covers for it.Exercise:Write an algorithm to compute Fibonacci series using DP.The implementation of fibonacci series in Python using both Top down and Bottom approach can be found at – https://youtu.be/OEtW3Fmmrpk

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