chore(CPlusPlus): add reverse stack (#351)

Co-authored-by: Ujjwal <75884061+UG-SEP@users.noreply.github.com>

algorithms/CPlusPlus/README.md

@@ -1,6 +1,7 @@
 # C++
 
 ## Arrays
+
 1. [Counting Inversions](Arrays/counting-inversions.cpp)
 2. [Dutch Flag Algorithm](Arrays/dutch-flag-algo.cpp)
 3. [Left Rotation](Arrays/left-rotation.cpp)
@@ -12,23 +13,27 @@
 9. [Fractional Knapsack](Arrays/fractional-knapsack.cpp)
 
 ## Graphs
+
 1. [Bellman Ford Algorithm](Graphs/bellmam-ford.cpp)
 2. [kruskal Algorithm](Graphs/kruskal-algorithm.cpp)
 3. [Breadth First Search](Graphs/breadth-first-search.cpp)
 
 ## Multiplication
+
 1. [Karatsuba](Multiplication/karatsuba.cpp)
 
 ## Linked Lists
+
 1. [All possible insertions](Linked-Lists/all-possible-insertion.cpp)
 2. [Singly linked lists](Linked-Lists/singly.cpp)
 3. [doubley linked lists](Linked-Lists/doubly.cpp)
 4. [Circular linked lists](Linked-Lists/circular.cpp)
 5. [Reversing a linked lists](Linked-Lists/reverse.cpp)
-5. [Merging two sorted linked lists](Linked-Lists/merge.cpp)
+6. [Merging two sorted linked lists](Linked-Lists/merge.cpp)
 6. [Reorder List](Linked-Lists/Reorder-List.cpp)
 
 ## Searching
+
 1. [Linear Search](Searching/linear-search.cpp)
 2. [Jump Search](Searching/jump-search.cpp)
 3. [Binary Search](Searching/binary-search.cpp)
@@ -39,10 +44,16 @@
 8. [Exponential Search](Searching/exponential-search.cpp)
 
 ## Stacks
+
 1. [Balancing Parenthesis](Stacks/balanced-parenthesis.cpp)
-2. [Stack using Array](Stacks/stack-using-array.cpp)
+
+2. [Reversing Stack](Stacks/reverse-stack.cpp)
+
+3. [Stack using Array](Stacks/stack-using-array.cpp)
+
 
 ## Sorting
+
 1. [Bubble Sort](Sorting/bubble-sort.cpp)
 2. [Insertion Sort](Sorting/insertion-sort.cpp)
 3. [Quicksort](Sorting/quick-sort.cpp)
@@ -58,6 +69,7 @@
 13. [Merge Sort](Sorting/merge-sort.cpp)
 
 ## Strings
+
 1. [Rabin-Karp pattern search algo](Strings/rabin-karp.cpp)
 2. [All subsequence of a string (Recursion) ](Strings/sequence.cpp)
 3. [String reversal](Strings/string-reverse.cpp)
@@ -65,6 +77,7 @@
 5. [Anagram check](Strings/anagram.cpp)
 
 ## Trees
+
 1. [Creating Binary Tree](Trees/build-binary-tree.cpp)
 2. [Counting and finding sum of all the nodes in BST](Trees/count-and-sum-of-nodes-in-binary-tree.cpp)
 3. [Level Order Traversal](Trees/level-order-traversal.cpp)
@@ -76,6 +89,7 @@
 9. [Min Heap](Trees/min-heap.cpp)
 
 # Maths
+
 1. [Kaprekar Number](Maths/Kaprekar-number.cpp)
 2. [Prime Number](Maths/prime-check.cpp)
 3. [Prime Sieve](Maths/prime-sieve.cpp)

algorithms/CPlusPlus/Stacks/reverse-stack.cpp

@@ -0,0 +1,73 @@
+/* the idea is to hold all items in a call stack until the stack becomes empty.
+Then insert each item in the call stack at the bottom of the stack. */
+
+#include <iostream>
+#include <stack>
+using namespace std;
+
+// Recursive function to insert an item to bottom of a given stack
+void insertInBottom(stack<int> &s, int item)
+{
+    // base case: if the stack is empty, insert given item at bottom
+    if (s.empty()) {
+        s.push(item);
+        return;
+    }
+
+    // Pop all items from the stack and hold them in call stack
+    int top = s.top();
+    s.pop();
+    insertInBottom(s, item);
+
+    /*After the recursion completes , push each item in the call stack
+     to top of the stack*/
+    s.push(top);
+}
+
+// Recursive function to reverse a given stack
+void reverseStack(stack<int> &s)
+{
+    // base case: stack is empty
+    if (s.empty()) {
+        return;
+    }
+
+    // Pop all items from the stack and hold them in call stack
+    int item = s.top();
+    s.pop();
+    reverseStack(s);
+
+    // After the recursion unfolds, insert each item from the call stack to bottom of the stack
+    insertInBottom(s, item);
+}
+
+int main()
+{
+    stack<int> s;
+    int n;
+    cout<<"Enter the size of stack"<<"\n";
+    cin>>n;
+    for (int i = 1; i <= n; i++) {
+        int temp;
+        cout<<"enter element"<<"\n";
+        cin>>temp;
+        s.push(temp);
+    }
+
+    reverseStack(s);
+
+    cout << "Reversed stack is: ";
+    while (!s.empty()) {
+        cout << s.top() << ' ';
+        s.pop();
+    }
+    
+    /*what program does (reverses the stack by using recursion)
+    -> original stack: 1,2,3,4,5
+    -> reversed stack: 5,4,3,2,1*/
+    
+    // time complexity : O(n^2)
+
+
+    return 0;
+}