What is Catalan number ? The Catalan number belongs to the domain of combinatorial mathematics. It is a sequence of natural numbers such that: 1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, 742900, 2674440, 9694845, 35357670, 129644790, 477638700, 1767263190, ... The sequence appears in counting problems. Wikipedia has the details about the sequence: Catalan Number.

The algorithm of the Catalan number calculation is not difficult. Once the problem is identified as the Catalan number, the solution will come up relatively easy. However, something not easy is to identify it is the Catalan number problem.

Various types of counting problems

The Wikipedia article mentions various types of counting problems which are solved by the Catalan number. One more great resource is “Catalan Numbers” (http://www.geometer.org/mathcircles/catalan.pdf). The document refers to multiple counting problems as well as how to solve those. For a reference, it’s worth writing a memo what kind of problems are out there.

1. Balanced parentheses

   Problem: Given n pairs of parentheses, how many patterns exist to create valid (balanced) combinations of parentheses.

   Example:

    n  counts  patterns
    0  1       *
    1  1       ()
    2  2       (()) ()()
    3  5       ((())) ()(()) ()()() (())() (()())
   

   Note: This is a basic pattern of the Catalan number.

2. Dyck words

   Problem: Given n, how many patterns exist to create words of n Xs and n Ys such that in every prefix of the word frequency(‘X’) ≥ frequency(‘Y’)

   Example:

    n  counts  patterns
    0  1       *
    1  1       XY
    2  2       XXYY XYXY
    3  5       XXXYYY XYXXYY XYXYXY XXYYXY XXYXYY
   

   Note: If X and Y are mapped ( and ) respectively, the problem is the same as the balanced parentheses.

3. Mountain Ranges

   Problem: Given n pairs of sticks, how many patterns exist to create mountains.

    n  counts  patterns
    0  1       *
    1  1       /\
    2  2       /\
              /  \  /\/\
    3  5        /\
               /  \     /\             /\      /\/\
              /    \  /\/  \  /\/\/\  /  \/\  /    \
   

   Note: If / and \ are mapped to ( and ) respectively, the problem is the same as the balanced parentheses.

4. Binary Search Trees

   Problem: Given n, how many unique binary search tress exist to create node value 1 to n.

   Example:

    n  counts  patterns
    0  1       *
    1  1       1
    2  2         2  1
                /    \
               1      2
    3  5           3    3     2    1    1
                  /    /     / \    \    \
                2    1     1   3    3    2
               /      \            /      \
              1        2          2        3
   

5. Diagonal Paths

   Problem: Given n x n grid, how many paths of length 2n exist from bottom left to upper right corner.

   Example:

    n  counts  patterns
    0  1       *
    1  1        ↑
               →
    2  2         ↑       ↑
                 ↑    ↑ →
              → →    →
    3  5           ↑        ↑         ↑        ↑          ↑
                   ↑        ↑      ↑ →      ↑ →           ↑
                   ↑   ↑ → →    ↑ →         ↑         ↑ →
              → → →   →        →         → →      → →
   

   Note: If → and ↑ are mapped to ( and ), the problem is the same as the balanced parentheses.

6. Multiplication orderings

   Problem: Given n+1 numbers to multiply together, how many patterns exist to make multiplication precedences without changing the order of numbers.

   Example:

    n  counts  patterns
    0  1       (a)
    1  1       (ab)
    2  2       ((ab)c) (a(bc))
    3  5       (((ab)c)d) ((ab)(cd)) ((a(bc))d) (a((bc)d)) (a(b(cd)))
   

7. Polygon Triangulation

   Problem: Given n+2 sides of convex polygon, how many patterns exist to cut into triangles of non-crossing line segments.

8. Handshaking

   Problem: Given 2n people seated around the round table, how many patterns exists to shake hands without crossing arms.

Code Example

For all problems above, the number of patterns can be found the code below:

The first numPatterns method runs faster if the problem asks only one number such as:

• leetcode: Unique Binary Search Trees
• topcoder: Hands Shaking

However, when problem asks accumulation of patterns such as:

• HackerRank: Popsicle Stick Mountains

the second DP solution works to avoid repetition.

Resources

• Wikipedia: Catalan number
• Tom Davis: Catalan Numbers
• GeeksforGeeks: Program for nth Catalan Number