Check if the given push and pop sequences of Stack is valid or not

Given push[] and pop[] sequences with distinct values. The task is to check if this could have been the result of a sequence of push and pop operations on an initially empty stack. Return “True” if it otherwise returns “False”.

Examples:

Input: pushed = { 1, 2, 3, 4, 5 }, popped = { 4, 5, 3, 2, 1 }
Output: True
Following sequence can be performed:
push(1), push(2), push(3), push(4), pop() -> 4,
push(5), pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Input: pushed = { 1, 2, 3, 4, 5 }, popped = { 4, 3, 5, 1, 2 }
Output: False
1 can't be popped before 2.

Approach: If the element X has been pushed to the stack then check if the top element in the pop[] sequence is X or not. If it is X then pop it right now else top of the push[] sequence will be changed and make the sequences invalid. So, similarly do the same for all the elements and check if the stack is empty or not in the last. If empty then print True else print False.

Below is the implementation of above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ implementation of above approach
#include <iostream>
#include <stack>
  
using namespace std;
  
// Function to check validity of stack sequence
bool validateStackSequence(int pushed[], int popped[], int len){
      
    // maintain count of popped elements
    int j = 0;
      
    // an empty stack
    stack <int> st;
    for(int i = 0; i < len; i++){
        st.push(pushed[i]);
          
        // check if appended value is next to be popped out
        while (!st.empty() && j < len && st.top() == popped[j]){
            st.pop();
            j++;
        }
    }
      
    return j == len;
}
  
// Driver code
int main()
{
   int pushed[] = {1, 2, 3, 4, 5};
   int popped[] = {4, 5, 3, 2, 1};
   int len = sizeof(pushed)/sizeof(pushed[0]);
     
   cout << (validateStackSequence(pushed, popped, len) ? "True" : "False");
     
   return 0;
}
  
// This code is contributed by Rituraj Jain

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java program for above implementation
import java.util.*;
  
class GfG 
{
  
    // Function to check validity of stack sequence 
    static boolean validateStackSequence(int pushed[],
                                        int popped[], int len) 
    {
  
        // maintain count of popped elements 
        int j = 0;
  
        // an empty stack 
        Stack<Integer> st = new Stack<>();
        for (int i = 0; i < len; i++) 
        {
            st.push(pushed[i]);
  
            // check if appended value 
            // is next to be popped out 
            while (!st.empty() && j < len && 
                    st.peek() == popped[j]) 
            {
                st.pop();
                j++;
            }
        }
  
        return j == len;
    }
  
    // Driver code 
    public static void main(String[] args) 
    {
        int pushed[] = {1, 2, 3, 4, 5};
        int popped[] = {4, 5, 3, 2, 1};
        int len = pushed.length;
  
        System.out.println((validateStackSequence(pushed, popped, len) ? "True" : "False"));
    }
}
  
/* This code contributed by PrinciRaj1992 */

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Function to check validity of stack sequence
def validateStackSequence(pushed, popped):
    # maintain count of popped elements
    j = 0
  
    # an empty stack
    stack = []
  
    for x in pushed:
        stack.append(x)
  
        # check if appended value is next to be popped out
        while stack and j < len(popped) and stack[-1] == popped[j]:
            stack.pop()
            j = j + 1
  
    return j == len(popped)
  
  
# Driver code
pushed = [1, 2, 3, 4, 5]
popped = [4, 5, 3, 2, 1]
print(validateStackSequence(pushed, popped))

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program for above implementation
using System; 
using System.Collections.Generic; 
  
class GfG 
{
  
    // Function to check validity of stack sequence 
    static bool validateStackSequence(int []pushed,
                                        int []popped, int len) 
    {
  
        // maintain count of popped elements 
        int j = 0;
  
        // an empty stack 
        Stack<int> st = new Stack<int>();
        for (int i = 0; i < len; i++) 
        {
            st.Push(pushed[i]);
  
            // check if appended value 
            // is next to be popped out 
            while (st.Count != 0 && j < len && 
                    st.Peek() == popped[j]) 
            {
                st.Pop();
                j++;
            }
        }
  
        return j == len;
    }
  
    // Driver code 
    public static void Main(String[] args) 
    {
        int []pushed = {1, 2, 3, 4, 5};
        int []popped = {4, 5, 3, 2, 1};
        int len = pushed.Length;
  
        Console.WriteLine((validateStackSequence(pushed, 
                        popped, len) ? "True" : "False"));
    }
}
  
// This code contributed by Rajput-Ji

chevron_right


PHP

Output:

True

Time Complexity: O(N), where N is size of stack.



My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.





Article Tags :
Practice Tags :


2


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.