Reverse a stack using recursion
Write a program to reverse a stack using recursion. You are not allowed to use loop constructs like while, for..etc, and you can only use the following ADT functions on Stack S:
isEmpty(S)
push(S)
pop(S)
The idea of the solution is to hold all values in Function Call Stack until the stack becomes empty. When the stack becomes empty, insert all held items one by one at the bottom of the stack.
For example, let the input stack be
1 <-- top 2 3 4
First 4 is inserted at the bottom. 4 <-- top Then 3 is inserted at the bottom 4 <-- top 3 Then 2 is inserted at the bottom 4 <-- top 3 2 Then 1 is inserted at the bottom 4 <-- top 3 2 1
So we need a function that inserts at the bottom of a stack using the above given basic stack function.
void insertAtBottom((): First pops all stack items and stores the popped item in function call stack using recursion. And when stack becomes empty, pushes new item and all items stored in call stack.
void reverse(): This function mainly uses insertAtBottom() to pop all items one by one and insert the popped items at the bottom.
C++
// C++ code to reverse a // stack using recursion #include<bits/stdc++.h> using namespace std; // using std::stack for // stack implementation stack< char > st; // initializing a string to store // result of reversed stack string ns; // Below is a recursive function // that inserts an element // at the bottom of a stack. void insert_at_bottom( char x) { if (st.size() == 0) st.push(x); else { // All items are held in Function Call // Stack until we reach end of the stack // When the stack becomes empty, the // st.size() becomes 0, the above if // part is executed and the item is // inserted at the bottom char a = st.top(); st.pop(); insert_at_bottom(x); // push allthe items held in // Function Call Stack // once the item is inserted // at the bottom st.push(a); } } // Below is the function that // reverses the given stack using // insert_at_bottom() void reverse() { if (st.size()>0) { // Hold all items in Function // Call Stack until we // reach end of the stack char x = st.top(); st.pop(); reverse(); // Insert all the items held // in Function Call Stack // one by one from the bottom // to top. Every item is // inserted at the bottom insert_at_bottom(x); } } // Driver Code int main() { // push elements into // the stack st.push( '1' ); st.push( '2' ); st.push( '3' ); st.push( '4' ); cout<< "Original Stack" <<endl; // print the elements // of original stack cout<< "1" << " " << "2" << " " << "3" << " " << "4" <<endl; // function to reverse // the stack reverse(); cout<< "Reversed Stack" <<endl; // storing values of reversed // stack into a string for display while (!st.empty()) { char p=st.top(); st.pop(); ns+=p; } //display of reversed stack cout<<ns[3]<< " " <<ns[2]<< " " <<ns[1]<< " " <<ns[0]<<endl; return 0; } // This code is contributed by Gautam Singh |
C
// C program to reverse a // stack using recursion #include<stdio.h> #include<stdlib.h> #define bool int // structure of a stack node struct sNode { char data; struct sNode *next; }; // Function Prototypes void push( struct sNode** top_ref, int new_data); int pop( struct sNode** top_ref); bool isEmpty( struct sNode* top); void print( struct sNode* top); // Below is a recursive function // that inserts an element // at the bottom of a stack. void insertAtBottom( struct sNode** top_ref, int item) { if (isEmpty(*top_ref)) push(top_ref, item); else { // Hold all items in Function Call // Stack until we reach end of the // stack. When the stack becomes // empty, the isEmpty(*top_ref)becomes // true, the above if part is executed // and the item is inserted at the bottom int temp = pop(top_ref); insertAtBottom(top_ref, item); // Once the item is inserted // at the bottom, push all // the items held in Function // Call Stack push(top_ref, temp); } } // Below is the function that // reverses the given stack using // insertAtBottom() void reverse( struct sNode** top_ref) { if (!isEmpty(*top_ref)) { // Hold all items in Function // Call Stack until we // reach end of the stack int temp = pop(top_ref); reverse(top_ref); // Insert all the items (held in // Function Call Stack) // one by one from the bottom // to top. Every item is // inserted at the bottom insertAtBottom(top_ref, temp); } } // Driver Code int main() { struct sNode *s = NULL; push(&s, 4); push(&s, 3); push(&s, 2); push(&s, 1); printf ( "\n Original Stack " ); print(s); reverse(&s); printf ( "\n Reversed Stack " ); print(s); return 0; } // Function to check if // the stack is empty bool isEmpty( struct sNode* top) { return (top == NULL)? 1 : 0; } // Function to push an item to stack void push( struct sNode** top_ref, int new_data) { // allocate node struct sNode* new_node = ( struct sNode*) malloc ( sizeof ( struct sNode)); if (new_node == NULL) { printf ( "Stack overflow \n" ); exit (0); } // put in the data new_node->data = new_data; // link the old list // off the new node new_node->next = (*top_ref); // move the head to // point to the new node (*top_ref) = new_node; } // Function to pop an item from stack int pop( struct sNode** top_ref) { char res; struct sNode *top; // If stack is empty then error if (*top_ref == NULL) { printf ( "Stack overflow \n" ); exit (0); } else { top = *top_ref; res = top->data; *top_ref = top->next; free (top); return res; } } // Function to print a // linked list void print( struct sNode* top) { printf ( "\n" ); while (top != NULL) { printf ( " %d " , top->data); top = top->next; } } |
Java
// Java code to reverse a // stack using recursion import java.util.Stack; class Test { // using Stack class for // stack implementation static Stack<Character> st = new Stack<>(); // Below is a recursive function // that inserts an element // at the bottom of a stack. static void insert_at_bottom( char x) { if (st.isEmpty()) st.push(x); else { // All items are held in Function // Call Stack until we reach end // of the stack. When the stack becomes // empty, the st.size() becomes 0, the // above if part is executed and // the item is inserted at the bottom char a = st.peek(); st.pop(); insert_at_bottom(x); // push allthe items held // in Function Call Stack // once the item is inserted // at the bottom st.push(a); } } // Below is the function that // reverses the given stack using // insert_at_bottom() static void reverse() { if (st.size() > 0 ) { // Hold all items in Function // Call Stack until we // reach end of the stack char x = st.peek(); st.pop(); reverse(); // Insert all the items held // in Function Call Stack // one by one from the bottom // to top. Every item is // inserted at the bottom insert_at_bottom(x); } } // Driver Code public static void main(String[] args) { // push elements into // the stack st.push( '1' ); st.push( '2' ); st.push( '3' ); st.push( '4' ); System.out.println( "Original Stack" ); System.out.println(st); // function to reverse // the stack reverse(); System.out.println( "Reversed Stack" ); System.out.println(st); } } |
Python3
# Python program to reverse a # stack using recursion # Below is a recursive function # that inserts an element # at the bottom of a stack. def insertAtBottom(stack, item): if isEmpty(stack): push(stack, item) else : temp = pop(stack) insertAtBottom(stack, item) push(stack, temp) # Below is the function that # reverses the given stack # using insertAtBottom() def reverse(stack): if not isEmpty(stack): temp = pop(stack) reverse(stack) insertAtBottom(stack, temp) # Below is a complete running # program for testing above # functions. # Function to create a stack. # It initializes size of stack # as 0 def createStack(): stack = [] return stack # Function to check if # the stack is empty def isEmpty( stack ): return len (stack) = = 0 # Function to push an # item to stack def push( stack, item ): stack.append( item ) # Function to pop an # item from stack def pop( stack ): # If stack is empty # then error if (isEmpty( stack )): print ( "Stack Underflow " ) exit( 1 ) return stack.pop() # Function to print the stack def prints(stack): for i in range ( len (stack) - 1 , - 1 , - 1 ): print (stack[i], end = ' ' ) print () # Driver Code stack = createStack() push( stack, str ( 4 ) ) push( stack, str ( 3 ) ) push( stack, str ( 2 ) ) push( stack, str ( 1 ) ) print ( "Original Stack " ) prints(stack) reverse(stack) print ( "Reversed Stack " ) prints(stack) # This code is contributed by Sunny Karira |
C#
// C# code to reverse a // stack using recursion using System; using System.Collections; public class GFG { // using Stack class for // stack implementation static Stack st = new Stack(); // Below is a recursive function // that inserts an element // at the bottom of a stack. static void insert_at_bottom( char x) { if (st.Count==0) st.Push(x); else { // All items are held in Function // Call Stack until we reach end // of the stack. When the stack becomes // empty, the st.size() becomes 0, the // above if part is executed and // the item is inserted at the bottom char a = ( char )st.Peek(); st.Pop(); insert_at_bottom(x); // push all the items held // in Function Call Stack // once the item is inserted // at the bottom st.Push(a); } } // Below is the function that // reverses the given stack using // insert_at_bottom() static void reverse() { if (st.Count > 0) { // Hold all items in Function // Call Stack until we // reach end of the stack char x = ( char )st.Peek(); st.Pop(); reverse(); // Insert all the items held // in Function Call Stack // one by one from the bottom // to top. Every item is // inserted at the bottom insert_at_bottom(x); } } // Driver Code public static void Main(String []args) { // push elements into // the stack st.Push( '1' ); st.Push( '2' ); st.Push( '3' ); st.Push( '4' ); Console.WriteLine( "Original Stack" ); foreach ( char i in st) { Console.WriteLine(i); } // function to reverse // the stack reverse(); Console.WriteLine( "Reversed Stack" ); foreach ( char i in st) { Console.WriteLine(i); } } } // This code is Contributed by Arnab Kundu |
Javascript
<script> // JavaScript code to reverse a // stack using recursion // using Stack class for // stack implementation let st = []; // Below is a recursive function // that inserts an element // at the bottom of a stack. function insert_at_bottom(x) { if (st.length==0) st.push(x); else { // All items are held in Function // Call Stack until we reach end // of the stack. When the stack becomes // empty, the st.size() becomes 0, the // above if part is executed and // the item is inserted at the bottom let a = st.pop(); insert_at_bottom(x); // push allthe items held // in Function Call Stack // once the item is inserted // at the bottom st.push(a); } } // Below is the function that // reverses the given stack using // insert_at_bottom() function reverse() { if (st.length > 0) { // Hold all items in Function // Call Stack until we // reach end of the stack let x = st.pop(); reverse(); // Insert all the items held // in Function Call Stack // one by one from the bottom // to top. Every item is // inserted at the bottom insert_at_bottom(x); } } // Driver Code // push elements into // the stack st.push( '1' ); st.push( '2' ); st.push( '3' ); st.push( '4' ); document.write( "Original Stack<br>" ); document.write(st.join( " " )+ "<br>" ); // function to reverse // the stack reverse(); document.write( "Reversed Stack<br>" ); document.write(st.join( " " )); // This code is contributed by avanitrachhadiya2155 </script> |
Output:
Original Stack 1 2 3 4 Reversed Stack 4 3 2 1
Time Complexity: This approach takes the worst time complexity of O(n^2),
Please write comments if you find any bug in above code/algorithm, or find other ways to solve the same problem.