topic 3: DP

how it works:
　　store some of the results by saving them: memoised result

 

 

 

 

1. 338 return a vector of counting bits

　　　　1. declare a vector of size: vector<int> v(n+1);

　　　　2. from bottom to up, find the patten.

2. 509 fib number

　　two ways:

　　　　1. bottom up

　　　　2. using recursion, up bottom