Add Catalan Numbers algorithm implementation (#191)

Author: iampratik13

mathematics/catalan_numbers.r

@@ -0,0 +1,110 @@
+# Catalan Numbers Implementation in R
+#
+# Catalan numbers form a sequence of natural numbers that occur in various counting problems.
+# The nth Catalan number is given by the formula: C(n) = (2n)! / ((n+1)! * n!)
+# They appear in problems like counting binary trees, valid parentheses combinations,
+# paths in a grid, and polygon triangulations.
+#
+# Time Complexity: O(n) for iterative approach, O(n^2) for recursive with memoization
+# Space Complexity: O(n) for memoization table
+
+# Function to calculate nth Catalan number using dynamic programming
+# @param n: Non-negative integer for which to calculate Catalan number
+# @return: The nth Catalan number
+catalan_dp <- function(n) {
+    # Base cases
+    if (n <= 1) {
+        return(1)
+    }
+    
+    # Initialize dp table
+    catalan <- numeric(n + 1)
+    catalan[1] <- 1  # C(0) = 1
+    catalan[2] <- 1  # C(1) = 1
+    
+    # Fill the table using the recurrence relation:
+    # C(n) = sum(C(i) * C(n-1-i)) for i from 0 to n-1
+    for (i in 2:n) {
+        catalan[i + 1] <- 0
+        for (j in 0:(i - 1)) {
+            catalan[i + 1] <- catalan[i + 1] + catalan[j + 1] * catalan[i - j]
+        }
+    }
+    
+    return(catalan[n + 1])
+}
+
+# Function to calculate nth Catalan number using direct formula
+# @param n: Non-negative integer for which to calculate Catalan number
+# @return: The nth Catalan number
+catalan_formula <- function(n) {
+    if (n <= 1) {
+        return(1)
+    }
+    
+    # Use the formula: C(n) = (2n)! / ((n+1)! * n!)
+    # Simplified to: C(n) = (2n choose n) / (n+1)
+    result <- 1
+    
+    # Calculate using the iterative formula to avoid large factorials
+    for (i in 0:(n - 1)) {
+        result <- result * (n + i + 1) / (i + 1)
+    }
+    
+    return(result / (n + 1))
+}
+
+# Function to generate first n Catalan numbers
+# @param n: Number of Catalan numbers to generate
+# @return: Vector containing first n Catalan numbers
+first_n_catalan <- function(n) {
+    if (n <= 0) {
+        return(numeric(0))
+    }
+    
+    result <- numeric(n)
+    
+    for (i in 1:n) {
+        result[i] <- catalan_dp(i - 1)  # Generate C(0) to C(n-1)
+    }
+    
+    return(result)
+}
+
+# Function to find applications of Catalan numbers
+# @param n: The index for which to show applications
+# @return: List of interpretations of the nth Catalan number
+catalan_applications <- function(n) {
+    cat_n <- catalan_dp(n)
+    
+    applications <- list(
+        value = cat_n,
+        interpretations = c(
+            paste("Number of ways to arrange", n, "pairs of parentheses"),
+            paste("Number of full binary trees with", n + 1, "leaves"),
+            paste("Number of ways to triangulate a convex polygon with", n + 2, "vertices"),
+            paste("Number of monotonic lattice paths from (0,0) to (n,n) not crossing y=x"),
+            paste("Number of ways to arrange", n, "non-attacking rooks on a triangular board")
+        )
+    )
+    
+    return(applications)
+}
+
+# Example usage:
+# # Calculate specific Catalan numbers
+# print(paste("5th Catalan number (DP):", catalan_dp(5)))
+# print(paste("5th Catalan number (Formula):", catalan_formula(5)))
+# 
+# # Generate first 10 Catalan numbers
+# first_10 <- first_n_catalan(10)
+# print("First 10 Catalan numbers:")
+# print(first_10)
+# 
+# # Show applications of 4th Catalan number
+# apps <- catalan_applications(4)
+# print(paste("C(4) =", apps$value))
+# print("Applications:")
+# for (app in apps$interpretations) {
+#     print(app)
+# }