feat: Add reverse linked list algorithim (TheAlgorithms#1596)

* add: reverse linked list algorithim * updating DIRECTORY.md * clang-format and clang-tidy fixes for 5cac47c * Update data_structures/reverse_a_linked_list.cpp Co-authored-by: David Leal <[email protected]> * Update data_structures/reverse_a_linked_list.cpp Co-authored-by: David Leal <[email protected]> * Update the description in data_structures/reverse_a_linked_list.cpp Co-authored-by: David Leal <[email protected]> * clang-format and clang-tidy fixes for 19b4a93 * Updating data_structures/reverse_a_linked_list.cpp Co-authored-by: David Leal <[email protected]> * add:test for the reversing algorithim * fix: synatx changes * docs :added an external link for explanation of the algorithim * fix: suggested changes * fix: chamges in the code * docs : changes required in the docs * docs :fix indentation * fix : ouptut statement changes * fix: clang-tidy warnings * formatting filenames f769503 * updating DIRECTORY.md * clang-format and clang-tidy fixes for f769503 * fix : display function removed * updating DIRECTORY.md * clang-format and clang-tidy fixes for 38547be * fix: changed the data types * clang-format and clang-tidy fixes for 3b2e68c * docs : documentation fixes Co-authored-by: David Leal <[email protected]> * docs : documentation changes Co-authored-by: David Leal <[email protected]> * clang-format and clang-tidy fixes for 57953be * docs: fix changes Co-authored-by: David Leal <[email protected]> * clang-format and clang-tidy fixes for db8e74a * fix : display changes Co-authored-by: ERR ! <[email protected]> * fix : added try block * fix: try-catch block * fix :merge conflict * docs:documentation changes * added more efficient test cases * clang-format and clang-tidy fixes for 83ee6e4 * Update data_structures/reverse_a_linked_list.cpp Co-authored-by: David Leal <[email protected]> Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com> Co-authored-by: David Leal <[email protected]> Co-authored-by: Abhinn Mishra <[email protected]> Co-authored-by: ERR ! <[email protected]>
shiv-tyagi · Sep 27, 2021 · f7a5aec · f7a5aec
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diff --git a/DIRECTORY.md b/DIRECTORY.md
@@ -52,6 +52,7 @@
   * [Queue Using Linkedlist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_linkedlist.cpp)
   * [Queue Using Two Stacks](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_two_stacks.cpp)
   * [Rb Tree](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/rb_tree.cpp)
+  * [Reverse A Linked List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/reverse_a_linked_list.cpp)
   * [Skip List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/skip_list.cpp)
   * [Sparse Table](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/sparse_table.cpp)
   * [Stack](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack.h)

diff --git a/data_structures/reverse_a_linked_list.cpp b/data_structures/reverse_a_linked_list.cpp
@@ -0,0 +1,208 @@
+/**
+ * @file
+ * @brief Implementation of [Reversing
+ * a single linked list](https://simple.wikipedia.org/wiki/Linked_list)
+ * @details
+ * The linked list is a data structure used for holding a sequence of
+ * values, which can be added, displayed, reversed, or removed.
+ * ### Algorithm
+ * Values can be added by iterating to the end of a list (by following
+ * the pointers) starting from the first link. Whichever link points to null
+ * is considered the last link and is pointed to the new value.
+ *
+ * Linked List can be reversed by using 3 pointers: current, previous, and
+ * next_node; we keep iterating until the last node. Meanwhile, before changing
+ * to the next of current, we store it in the next_node pointer, now we store
+ * the prev pointer in the current of next, this is where the actual reversal
+ * happens. And then we move the prev and current pointers one step forward.
+ * Then the head node is made to point to the last node (prev pointer) after
+ * completion of an iteration.
+
+ * [A graphic explanation and view of what's happening behind the
+ *scenes](https://drive.google.com/file/d/1pM5COF0wx-wermnNy_svtyZquaCUP2xS/view?usp=sharing)
+ */
+
+#include <cassert>   /// for assert
+#include <iostream>  /// for I/O operations
+#include <memory>    /// for dynamic memory
+#include <new>       /// for managing  dynamic storage
+
+/**
+ * @namespace data_structures
+ * @brief Data Structures algorithms
+ */
+namespace data_structures {
+/**
+ * @namespace linked_list
+ * @brief Functions for singly linked list algorithm
+ */
+namespace linked_list {
+/**
+ * A Node class containing a value and pointer to another link
+ */
+class Node {
+ public:
+    int32_t val;  /// value of the current link
+    Node *next;   /// pointer to the next value on the list
+};
+
+/**
+ * A list class containing a sequence of links
+ */
+class list {
+ private:
+    Node *head;  // link before the actual first element
+ public:
+    /**
+     * List constructor. Initializes the first link.
+     */
+    list() {
+        head = nullptr;  // Initialize the first link
+    }
+    bool isEmpty();
+    void insert(int32_t new_elem);
+    void reverseList();
+    void display();
+    int32_t top();
+    int32_t last();
+    int32_t traverse(int32_t index);
+};
+
+/**
+ * @brief Utility function that checks if the list is empty
+ * @returns true if the list is empty
+ * @returns false if the list is not empty
+ */
+bool list::isEmpty() { return head == nullptr; }
+
+/**
+ * @brief Utility function that adds a new element at the end of the list
+ * @param new_elem element be added at the end of the list
+ */
+void list::insert(int32_t n) {
+    try {
+        Node *new_node = new Node();
+        Node *temp = nullptr;
+        new_node->val = n;
+        new_node->next = nullptr;
+        if (isEmpty()) {
+            head = new_node;
+        } else {
+            temp = head;
+            while (temp->next != nullptr) {
+                temp = temp->next;
+            }
+            temp->next = new_node;
+        }
+    } catch (std::bad_alloc &exception) {
+        std::cerr << "bad_alloc detected: " << exception.what() << "\n";
+    }
+}
+
+/**
+ * @brief Utility function for reversing a list
+ * @brief Using the current, previous, and next pointer.
+ * @returns void
+ */
+void list::reverseList() {
+    Node *curr = head;
+    Node *prev = nullptr, *next_node = nullptr;
+    while (curr != nullptr) {
+        next_node = curr->next;
+        curr->next = prev;
+        prev = curr;
+        curr = next_node;
+    }
+    head = prev;
+}
+
+/**
+ * @brief Utility function to find the top element of the list
+ * @returns the top element of the list
+ */
+int32_t list::top() {
+    try {
+        if (!isEmpty()) {
+            return head->val;
+        }
+    } catch (const std::exception &e) {
+        std::cerr << "List is empty" << e.what() << '\n';
+    }
+}
+/**
+ *  @brief Utility function to find the last element of the list
+ *  @returns the last element of the list
+ */
+int32_t list::last() {
+    try {
+        if (!isEmpty()) {
+            Node *t = head;
+            while (t->next != nullptr) {
+                t = t->next;
+            }
+            return t->val;
+        }
+    } catch (const std::exception &e) {
+        std::cerr << "List is empty" << e.what() << '\n';
+    }
+}
+/**
+ *  @brief Utility function to find the i th element of the list
+ *  @returns the i th element of the list
+ */
+int32_t list::traverse(int index) {
+    Node *current = head;
+
+    int count = 0;
+    while (current != nullptr) {
+        if (count == index) {
+            return (current->val);
+        }
+        count++;
+        current = current->next;
+    }
+
+    /* if we get to this line,the caller was asking for a non-existent element
+    so we assert fail */
+    assert(0);
+}
+
+}  // namespace linked_list
+}  // namespace data_structures
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    data_structures::linked_list::list L;
+    // 1st test
+    L.insert(11);
+    L.insert(12);
+    L.insert(15);
+    L.insert(10);
+    L.insert(-12);
+    L.insert(-20);
+    L.insert(18);
+    assert(L.top() == 11);
+    assert(L.last() == 18);
+    L.reverseList();
+    // Reversal Testing
+    assert(L.top() == 18);
+    assert(L.traverse(1) == -20);
+    assert(L.traverse(2) == -12);
+    assert(L.traverse(3) == 10);
+    assert(L.traverse(4) == 15);
+    assert(L.traverse(5) == 12);
+    assert(L.last() == 11);
+    std::cout << "All tests have successfully passed!" << std::endl;
+}
+
+/**
+ * @brief Main function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
+    return 0;
+}