javascript-algorithms
Pascal’s Triangle
In mathematics, Pascal’s triangle is a triangular array of the binomial coefficients.
The rows of Pascal’s triangle are conventionally enumerated
starting with row n = 0 at the top (the 0th row). The
entries in each row are numbered from the left beginning
with k = 0 and are usually staggered relative to the
numbers in the adjacent rows. The triangle may be constructed
in the following manner: In row 0 (the topmost row), there
is a unique nonzero entry 1. Each entry of each subsequent
row is constructed by adding the number above and to the
left with the number above and to the right, treating blank
entries as 0. For example, the initial number in the
first (or any other) row is 1 (the sum of 0 and 1),
whereas the numbers 1 and 3 in the third row are added
to produce the number 4 in the fourth row.
Formula
The entry in the nth row and kth column of Pascal’s
triangle is denoted 
With this notation, the construction of the previous paragraph may be written as follows:
for any non-negative integer n and any
integer k between 0 and n, inclusive.
Calculating triangle entries in O(n) time
We know that i-th entry in a line number lineNumber is
Binomial Coefficient C(lineNumber, i) and all lines start
with value 1. The idea is to
calculate C(lineNumber, i) using C(lineNumber, i-1). It
can be calculated in O(1) time using the following:
C(lineNumber, i) = lineNumber! / ((lineNumber - i)! * i!)
C(lineNumber, i - 1) = lineNumber! / ((lineNumber - i + 1)! * (i - 1)!)
We can derive following expression from above two expressions:
C(lineNumber, i) = C(lineNumber, i - 1) * (lineNumber - i + 1) / i
So C(lineNumber, i) can be calculated
from C(lineNumber, i - 1) in O(1) time.