Count the number of pop operations on stack to get each element of the array

Prerequisite: Stack, Hashing

Given a stack of N numbers and an array of numbers. Count the numbers of pop operations required to get each element of the array. Once an element is popped then its not pushed back again. Assume that the all the elements from the array present inside the stack initially.

Examples:

Input: N = 5
Stack: 6 4 3 2 1
Array: 6 3 4 1 2
Output: 1 2 0 2 0

The 1st element of the stack is the same as the array elements. So to get 6, one pop is required i.e. pop 6 from the stack. To get 3, 2 elements will be popped i.e. 4 and 3. To get 4, 4 was already popped, thus we won’t pop more elements. Similarly, to get 1, we will pop 2 elements and for the 2, no pop count will be added.



Approach: This question can be solved easily by using a stack. We will keep popping the elements till we find the element we are searching. The only hurdle is that how to handle the case when the element is already popped and is not present in the stack. For that, we will maintain a hash map. As we pop an element from the stack we will insert that element in the hash map so that if the element comes later in the array we will first check if its present inside the hash maps or in other words has been popped out from the stack previously. Otherwise, we will know it’s present inside the stack and we will start popping the elements till we find the required number.

Below is the implementation of above approach:

C++

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// C++ program to implement above approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the count
void countEle(stack<int>& s, int a[], int N)
{
    // Hashmap to store all the elements
    // which are popped once.
    unordered_map<int, bool> mp;
    for (int i = 0; i < N; ++i) {
        int num = a[i];
  
        // Check if the number is present
        // in the hashmap Or in other words
        // been popped out from the stack before.
        if (mp.find(num) != mp.end())
            cout << "0 ";
  
        else {
            int cnt = 0;
  
            // Keep popping the elements
            // while top is not equal to num
            while (s.top() != num) {
                mp[s.top()] = true;
                s.pop();
                cnt++;
            }
            // Pop the top ie. equal to num
            s.pop();
            cnt++;
  
            // Print the number of elements popped.
            cout << cnt << " ";
        }
    }
}
  
// Driver code
int main()
{
    int N = 5;
  
    stack<int> s;
    s.push(1);
    s.push(2);
    s.push(3);
    s.push(4);
    s.push(6);
  
    int a[] = { 6, 3, 4, 1, 2 };
    countEle(s, a, N);
  
    return 0;
}

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Python3

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# Python3 program to implement above approach 
  
# Function to find the count 
def countEle(s, a, N): 
   
    # Hashmap to store all the elements 
    # which are popped once. 
    mp = {} 
    for i in range(0, N):  
        num = a[i] 
  
        # Check if the number is present 
        # in the hashmap Or in other words 
        # been popped out from the stack before. 
        if num in mp: 
            print("0", end = " "
  
        else:
            cnt = 0 
  
            # Keep popping the elements 
            # while top is not equal to num 
            while s[-1] != num:
                mp[s.pop()] = True 
                cnt += 1
               
            # Pop the top ie. equal to num 
            s.pop()
            cnt += 1
  
            # Print the number of elements popped. 
            print(cnt, end = " "
  
# Driver code 
if __name__ == "__main__":
   
    N = 5 
    s = [] 
    s.append(1
    s.append(2
    s.append(3
    s.append(4
    s.append(6
  
    a = [6, 3, 4, 1, 2]  
    countEle(s, a, N) 
  
# This code is contributed by Rituraj Jain

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Output:

1 2 0 2 0

Time Complexity: O(N)



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