feat: add Two-Pointer Technique for linked lists (#283)

Author: Brijesh03032001

DIRECTORY.md

@@ -69,6 +69,7 @@
   * [Circular Linked List](https://github.com/TheAlgorithms/R/blob/HEAD/linked_list_algorithms/circular_linked_list.r)
   * [Doubly Linked List](https://github.com/TheAlgorithms/R/blob/HEAD/linked_list_algorithms/doubly_linked_list.r)
   * [Singly Linked List](https://github.com/TheAlgorithms/R/blob/HEAD/linked_list_algorithms/singly_linked_list.r)
+  * [Two-Pointer Technique (Fast and Slow Pointers)](https://github.com/TheAlgorithms/R/blob/HEAD/linked_list_algorithms/two_pointer_technique.r)
 
 ## Machine Learning
   * [Gradient Boosting](https://github.com/TheAlgorithms/R/blob/HEAD/machine_learning/gradient_boosting.r)

linked_list_algorithms/two_pointer_technique.r

@@ -0,0 +1,133 @@
+# Two-Pointer Technique (Fast and Slow Pointers / Tortoise and Hare)
+#
+# Template and practical application for singly linked lists using the
+# fast & slow pointer technique. This file provides:
+# - A minimal Node reference-class local to this file
+# - Generic helper: build list from a vector
+# - detect_cycle(head): returns TRUE/FALSE using tortoise & hare
+# - find_middle(head): returns the middle node's data (for even length returns the second middle)
+# - nth_from_end(head, n): returns the data of the Nth node from the end (1-based)
+#
+# Practical application demonstrated: finding the Nth node from the end.
+
+TwoPtrNode <- setRefClass("TwoPtrNode",
+  fields = list(
+    data = "ANY",
+    next_node = "ANY"
+  ),
+  methods = list(
+    initialize = function(data = NULL, next_node = NULL) {
+      .self$data <- data
+      .self$next_node <- next_node
+    },
+
+    print = function() {
+      cat("Node(data =", .self$data, ")\n")
+    }
+  )
+)
+
+# Build a singly linked list from an R vector of values. Returns the head node.
+build_list_from_vector <- function(vec) {
+  if (length(vec) == 0) return(NULL)
+  head <- TwoPtrNode$new(data = vec[1])
+  current <- head
+  if (length(vec) > 1) {
+    for (v in vec[-1]) {
+      new_node <- TwoPtrNode$new(data = v)
+      current$next_node <- new_node
+      current <- new_node
+    }
+  }
+  return(head)
+}
+
+# Convert linked list to vector for easy printing/inspection
+# This is defensive against cycles by truncating after `max_nodes` items.
+list_to_vector <- function(head, max_nodes = 1000) {
+  out <- c()
+  cur <- head
+  i <- 0
+  while (!is.null(cur) && i < max_nodes) {
+    out <- c(out, cur$data)
+    cur <- cur$next_node
+    i <- i + 1
+  }
+  if (!is.null(cur)) {
+    out <- c(out, "... (truncated or cycle detected)")
+  }
+  return(out)
+}
+
+# Detect cycle using fast and slow pointers (Tortoise & Hare)
+detect_cycle <- function(head) {
+  if (is.null(head)) return(FALSE)
+  slow <- head
+  fast <- head
+  while (!is.null(fast) && !is.null(fast$next_node)) {
+    slow <- slow$next_node
+    fast <- fast$next_node$next_node
+    if (identical(slow, fast)) return(TRUE)
+  }
+  return(FALSE)
+}
+
+# Find middle node using two pointers. For even-length lists this returns
+# the second middle (i.e., for 1->2->3->4 it returns 3).
+find_middle <- function(head) {
+  if (is.null(head)) return(NULL)
+  slow <- head
+  fast <- head
+  while (!is.null(fast) && !is.null(fast$next_node)) {
+    slow <- slow$next_node
+    fast <- fast$next_node$next_node
+  }
+  return(slow)
+}
+
+# Return the data of the Nth node from the end (1-based). Throws an error
+# if n is invalid or greater than list length.
+nth_from_end <- function(head, n) {
+  if (is.null(head)) stop("List is empty")
+  if (n <= 0) stop("n must be a positive integer")
+
+  fast <- head
+  # Advance fast by n steps
+  for (i in seq_len(n)) {
+    if (is.null(fast)) stop(sprintf("n (%d) is larger than the list length", n))
+    fast <- fast$next_node
+  }
+
+  slow <- head
+  # Move both until fast is NULL; slow will be at the Nth from end
+  while (!is.null(fast)) {
+    slow <- slow$next_node
+    fast <- fast$next_node
+  }
+  return(slow$data)
+}
+
+# ---- Practical demonstration ----
+if (sys.nframe() == 0) {
+  cat("Two-Pointer Technique demo\n")
+  head <- build_list_from_vector(1:7)
+  cat("List:", paste(list_to_vector(head), collapse = " -> "), "\n")
+
+  mid <- find_middle(head)
+  cat("Middle node data:", ifelse(is.null(mid), "NULL", mid$data), "\n")
+
+  n <- 2
+  nth <- nth_from_end(head, n)
+  cat(sprintf("%d-th node from the end: %s\n", n, nth))
+
+  cat("Cycle detected?", detect_cycle(head), "\n")
+
+  # Create a cycle for testing (connect tail to node with data=3)
+  tail <- head
+  while (!is.null(tail$next_node)) tail <- tail$next_node
+  cur <- head
+  while (!is.null(cur) && cur$data != 3) cur <- cur$next_node
+  if (!is.null(cur)) tail$next_node <- cur
+  cat("After creating a cycle (tail -> node with data 3):\n")
+  cat("Cycle detected?", detect_cycle(head), "\n")
+}