# Sort a stack using a temporary stack

• Difficulty Level : Medium
• Last Updated : 13 Jul, 2022

Given a stack of integers, sort it in ascending order using another temporary stack.

Examples:

```Input : [34, 3, 31, 98, 92, 23]
Output : [3, 23, 31, 34, 92, 98]

Input : [3, 5, 1, 4, 2, 8]
Output : [1, 2, 3, 4, 5, 8] ```
Recommended Practice

Algorithm:

1. Create a temporary stack say tmpStack.
2. While input stack is NOT empty do this:
• Pop an element from input stack call it temp
• while temporary stack is NOT empty and top of temporary stack is greater than temp,
pop from temporary stack and push it to the input stack
• push temp in temporary stack
3. The sorted numbers are in tmpStack

Here is a dry run of the above pseudo code.

```input: [34, 3, 31, 98, 92, 23]

Element taken out: 23
input: [34, 3, 31, 98, 92]
tmpStack: 

Element taken out: 92
input: [34, 3, 31, 98]
tmpStack: [23, 92]

Element taken out: 98
input: [34, 3, 31]
tmpStack: [23, 92, 98]

Element taken out: 31
input: [34, 3, 98, 92]
tmpStack: [23, 31]

Element taken out: 92
input: [34, 3, 98]
tmpStack: [23, 31, 92]

Element taken out: 98
input: [34, 3]
tmpStack: [23, 31, 92, 98]

Element taken out: 3
input: [34, 98, 92, 31, 23]
tmpStack: 

Element taken out: 23
input: [34, 98, 92, 31]
tmpStack: [3, 23]

Element taken out: 31
input: [34, 98, 92]
tmpStack: [3, 23, 31]

Element taken out: 92
input: [34, 98]
tmpStack: [3, 23, 31, 92]

Element taken out: 98
input: 
tmpStack: [3, 23, 31, 92, 98]

Element taken out: 34
input: [98, 92]
tmpStack: [3, 23, 31, 34]

Element taken out: 92
input: 
tmpStack: [3, 23, 31, 34, 92]

Element taken out: 98
input: []
tmpStack: [3, 23, 31, 34, 92, 98]

final sorted list: [3, 23, 31, 34, 92, 98]```

Implementation:

## C++

 `// C++ program to sort a stack using an` `// auxiliary stack.` `#include ` `using` `namespace` `std;`   `// This function return the sorted stack` `stack<``int``> sortStack(stack<``int``> &input)` `{` `    ``stack<``int``> tmpStack;`   `    ``while` `(!input.empty())` `    ``{` `        ``// pop out the first element` `        ``int` `tmp = input.top();` `        ``input.pop();`   `        ``// while temporary stack is not empty and top` `        ``// of stack is greater than temp` `        ``while` `(!tmpStack.empty() && tmpStack.top() > tmp)` `        ``{` `            ``// pop from temporary stack and push` `            ``// it to the input stack` `            ``input.push(tmpStack.top());` `            ``tmpStack.pop();` `        ``}`   `        ``// push temp in temporary of stack` `        ``tmpStack.push(tmp);` `    ``}`   `    ``return` `tmpStack;` `}`   `// main function` `int` `main()` `{` `    ``stack<``int``> input;` `    ``input.push(34);` `    ``input.push(3);` `    ``input.push(31);` `    ``input.push(98);` `    ``input.push(92);` `    ``input.push(23);`   `    ``// This is the temporary stack` `    ``stack<``int``> tmpStack = sortStack(input);` `    ``cout << ``"Sorted numbers are:\n"``;`   `    ``while` `(!tmpStack.empty())` `    ``{` `        ``cout << tmpStack.top()<< ``" "``;` `        ``tmpStack.pop();` `    ``}` `}`

## Java

 `// Java program to sort a stack using ` `// a auxiliary stack.` `import` `java.util.*;`   `class` `SortStack` `{` `    ``// This function return the sorted stack` `    ``public` `static` `Stack sortstack(Stack ` `                                             ``input)` `    ``{` `        ``Stack tmpStack = ``new` `Stack();` `        ``while``(!input.isEmpty())` `        ``{` `            ``// pop out the first element` `            ``int` `tmp = input.pop();` `        `  `            ``// while temporary stack is not empty and` `            ``// top of stack is greater than temp` `            ``while``(!tmpStack.isEmpty() && tmpStack.peek() ` `                                                 ``> tmp)` `            ``{` `                ``// pop from temporary stack and ` `                ``// push it to the input stack` `            ``input.push(tmpStack.pop());` `            ``}` `            `  `            ``// push temp in temporary of stack` `            ``tmpStack.push(tmp);` `        ``}` `        ``return` `tmpStack;` `    ``}` `    `  `    ``// Driver Code` `    ``public` `static` `void` `main(String args[])` `    ``{` `        ``Stack input = ``new` `Stack();` `        ``input.add(``34``);` `        ``input.add(``3``);` `        ``input.add(``31``);` `        ``input.add(``98``);` `        ``input.add(``92``);` `        ``input.add(``23``);` `    `  `        ``// This is the temporary stack` `        ``Stack tmpStack=sortstack(input);` `        ``System.out.println(``"Sorted numbers are:"``);` `    `  `        ``while` `(!tmpStack.empty())` `        ``{` `            ``System.out.print(tmpStack.pop()+``" "``);` `        ``} ` `    ``}` `}` `// This code is contributed by Danish Kaleem`

## Python3

 `# Python program to sort a ` `# stack using auxiliary stack.`   `# This function return the sorted stack` `def` `sortStack ( stack ):` `    ``tmpStack ``=` `createStack()` `    ``while``(isEmpty(stack) ``=``=` `False``):` `        `  `        ``# pop out the first element` `        ``tmp ``=` `top(stack)` `        ``pop(stack)`   `        ``# while temporary stack is not` `        ``# empty and top of stack is` `        ``# greater than temp` `        ``while``(isEmpty(tmpStack) ``=``=` `False` `and` `             ``int``(top(tmpStack)) > ``int``(tmp)):` `            `  `            ``# pop from temporary stack and` `            ``# push it to the input stack` `            ``push(stack,top(tmpStack))` `            ``pop(tmpStack)`   `        ``# push temp in temporary of stack` `        ``push(tmpStack,tmp)` `    `  `    ``return` `tmpStack`   `# Below is a complete running ` `# program for testing above` `# function.`   `# 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 get top ` `# item of stack` `def` `top( stack ):` `    ``p ``=` `len``(stack)` `    ``return` `stack[p``-``1``]`   `# 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``(``34``) )` `push( stack, ``str``(``3``) )` `push( stack, ``str``(``31``) )` `push( stack, ``str``(``98``) )` `push( stack, ``str``(``92``) )` `push( stack, ``str``(``23``) )`   `print``(``"Sorted numbers are: "``)` `sortedst ``=` `sortStack ( stack )` `prints(sortedst)`   `# This code is contributed by` `# Prasad Kshirsagar`

## C#

 `// C# program to sort a stack using ` `// a auxiliary stack. ` `using` `System;` `using` `System.Collections.Generic;`   `class` `GFG` `{` `// This function return the sorted stack ` `public` `static` `Stack<``int``> sortstack(Stack<``int``> input)` `{` `    ``Stack<``int``> tmpStack = ``new` `Stack<``int``>();` `    ``while` `(input.Count > 0)` `    ``{` `        ``// pop out the first element ` `        ``int` `tmp = input.Pop();`   `        ``// while temporary stack is not empty and ` `        ``// top of stack is greater than temp ` `        ``while` `(tmpStack.Count > 0 && tmpStack.Peek() > tmp)` `        ``{` `            ``// pop from temporary stack and ` `            ``// push it to the input stack ` `            ``input.Push(tmpStack.Pop());` `        ``}`   `        ``// push temp in temporary of stack ` `        ``tmpStack.Push(tmp);` `    ``}` `    ``return` `tmpStack;` `}`   `// Driver Code ` `public` `static` `void` `Main(``string``[] args)` `{` `    ``Stack<``int``> input = ``new` `Stack<``int``>();` `    ``input.Push(34);` `    ``input.Push(3);` `    ``input.Push(31);` `    ``input.Push(98);` `    ``input.Push(92);` `    ``input.Push(23);`   `    ``// This is the temporary stack ` `    ``Stack<``int``> tmpStack = sortstack(input);` `    ``Console.WriteLine(``"Sorted numbers are:"``);`   `    ``while` `(tmpStack.Count > 0)` `    ``{` `        ``Console.Write(tmpStack.Pop() + ``" "``);` `    ``}` `}` `}`   `// This code is contributed by Shrikant13`

## Javascript

 ``

Output

```Sorted numbers are:
98 92 34 31 23 3 ```

Time Complexity: O(n2) where n is the total number of integers in the given stack.
Auxiliary Space: O(n)

Microsoft

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