More info: http://en.wikipedia.org/wiki/N-gram
MapReduce in Python
Almost everyone has heard of Google’s MapReduce framework, but very few have ever hacked around with the idea of map and reduce. These two idioms are borrowed from functional programming, and form the basis of Google’s framework. Although Python is not a functional programming language, it has built-in support for both of these concepts.
A map operation involves taking a function f and applying it on a per-element basis to a given list L. For example, if f was the square-root function, then the map would take the square of each element in L. A reduce operation (also known as folding) is similar in that it also applies a function g to a given list L, but instead of isolating on each element, g systematically accumulates or collapses the contents of L into a single result. The canonical example of this would g performing a…
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Great post for implementing skip list in python.
Skip list is a probabilistic data structure that allows efficient search, insertion and removal operations. It was invented by William Pugh  in 1989.
Other structures that have efficient operations are self-balancing binary trees, such as AVL, Red-black and splay tree. But they are often considered difficult to implement.
On the other hand, skip lists are much like multiple linked lists with some randomization.
In the first level, we have a regular linked list with the elements sorted. Each element of this list has a probability $latex p$ to be also present in the level above. The second level will probably contain fewer elements and each of these elements will also have a chance $latex p$ to be on the third level, and so on. Figure 1 shows an example of a skip list.
We’ll implement a simple version of the skip list in python. To start, we define a…
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Original implementation from: http://www.laurentluce.com/posts/solving-mazes-using-python-simple-recursivity-and-a-search/