In a stack the command to access nth element from the top of the stack will be * 1 point S[Top-1] S[Top+1] S[Top-n-1] None of the above code example
Design a stack with operations on middle element
How to implement a stack which will support following operations in O[1] time complexity?
1] push[] which adds an element to the top of stack.
2] pop[] which removes an element from top of stack.
3] findMiddle[] which will return middle element of the stack.
4] deleteMiddle[] which will delete the middle element.
Push and pop are standard stack operations.
The important question is, whether to use a linked list or array for the implementation of the stack?
Please note that we need to find and delete the middle element. Deleting an element from the middle is not O[1] for array. Also, we may need to move the middle pointer up when we push an element and move down when we pop[]. In a singly linked list, moving the middle pointer in both directions is not possible.
The idea is to use Doubly Linked List [DLL]. We can delete the middle element in O[1] time by maintaining mid pointer. We can move mid pointer in both directions using previous and next pointers.
Following is implementation of push[], pop[] and findMiddle[] operations. Implementation of deleteMiddle[] is left as an exercise. If there are even elements in stack, findMiddle[] returns the second middle element. For example, if stack contains {1, 2, 3, 4}, then findMiddle[] would return 3.
C++
|
/* C++ Program to implement a stack that supports findMiddle[] and deleteMiddle in O[1] time */ #include using namespace std; class myStack { struct Node { int num; Node *next; Node *prev; Node[int num] { this->num = num; } }; //Members of stack Node *head = NULL; Node *mid = NULL; int size = 0; public: void push[int data] { Node *temp = new Node[data]; if [size==0] { head = temp; mid = temp; size++; return; } head->next = temp; temp->prev = head; //update the pointers head = head->next; if [size%2==1] { mid = mid->next; } size++; } void pop[] { if [size!=0] { if [size==1] { head = NULL; mid = NULL; } else { head = head->prev; head->next = NULL; if [size%2==0] { mid = mid->prev; } } size--; } } int findMiddle[] { if [size==0] { return -1; } return mid->num; } void deleteMiddle[] { if [size!=0] { if [size==1] { head = NULL; mid = NULL; } else if [size==2] { head = head->prev; mid = mid->prev; head->next =NULL; } else { mid->next->prev = mid->prev; mid->prev->next = mid->next; if [size%2==0] { mid = mid->prev; } else { mid = mid->next; } } size--; } } }; int main[] { myStack st; st.push[11]; st.push[22]; st.push[33]; st.push[44]; st.push[55]; st.pop[]; st.pop[]; st.pop[]; coutmid = new_DLLNode; } else { ms->head->prev = new_DLLNode; if [ms->count & 1] // Update mid if ms->count is odd ms->mid = ms->mid->prev; } /* move head to point to the new DLLNode */ ms->head = new_DLLNode; } /* Function to pop an element from stack */ int pop[struct myStack* ms] { /* Stack underflow */ if [ms->count == 0] { printf["Stack is empty\n"]; return -1; } struct DLLNode* head = ms->head; int item = head->data; ms->head = head->next; // If linked list doesn't become empty, update prev // of new head as NULL if [ms->head != NULL] ms->head->prev = NULL; ms->count -= 1; // update the mid pointer when we have even number of // elements in the stack, i,e move down the mid pointer. if [![[ms->count] & 1]] ms->mid = ms->mid->next; free[head]; return item; } // Function for finding middle of the stack int findMiddle[struct myStack* ms] { if [ms->count == 0] { printf["Stack is empty now\n"]; return -1; } return ms->mid->data; } // Driver program to test functions of myStack int main[] { /* Let us create a stack using push[] operation*/ struct myStack* ms = createMyStack[]; push[ms, 11]; push[ms, 22]; push[ms, 33]; push[ms, 44]; push[ms, 55]; push[ms, 66]; push[ms, 77]; printf["Item popped is %d\n", pop[ms]]; printf["Item popped is %d\n", pop[ms]]; printf["Middle Element is %d\n", findMiddle[ms]]; return 0; } |
Java
|
/* Java Program to implement a stack that supports findMiddle[] and deleteMiddle in O[1] time */ public class GFG { /* A Doubly Linked List Node */ class DLLNode { DLLNode prev; int data; DLLNode next; DLLNode[int d] { data = d; } } /* Representation of the stack data structure that supports findMiddle[] in O[1] time. The Stack is implemented using Doubly Linked List. It maintains pointer to head node, pointer to middle node and count of nodes */ class myStack { DLLNode head; DLLNode mid; int count; } /* Function to create the stack data structure */ myStack createMyStack[] { myStack ms = new myStack[]; ms.count = 0; return ms; } /* Function to push an element to the stack */ void push[myStack ms, int new_data] { /* allocate DLLNode and put in data */ DLLNode new_DLLNode = new DLLNode[new_data]; /* Since we are adding at the beginning, prev is always NULL */ new_DLLNode.prev = null; /* link the old list off the new DLLNode */ new_DLLNode.next = ms.head; /* Increment count of items in stack */ ms.count += 1; /* Change mid pointer in two cases 1] Linked List is empty 2] Number of nodes in linked list is odd */ if [ms.count == 1] { ms.mid = new_DLLNode; } else { ms.head.prev = new_DLLNode; if [[ms.count % 2] != 0] // Update mid if ms->count is odd ms.mid = ms.mid.prev; } /* move head to point to the new DLLNode */ ms.head = new_DLLNode; } /* Function to pop an element from stack */ int pop[myStack ms] { /* Stack underflow */ if [ms.count == 0] { System.out.println["Stack is empty"]; return -1; } DLLNode head = ms.head; int item = head.data; ms.head = head.next; // If linked list doesn't become empty, update prev // of new head as NULL if [ms.head != null] ms.head.prev = null; ms.count -= 1; // update the mid pointer when we have even number // of elements in the stack, i,e move down the mid // pointer. if [ms.count % 2 == 0] ms.mid = ms.mid.next; return item; } // Function for finding middle of the stack int findMiddle[myStack ms] { if [ms.count == 0] { System.out.println["Stack is empty now"]; return -1; } return ms.mid.data; } // Driver program to test functions of myStack public static void main[String args[]] { GFG ob = new GFG[]; myStack ms = ob.createMyStack[]; ob.push[ms, 11]; ob.push[ms, 22]; ob.push[ms, 33]; ob.push[ms, 44]; ob.push[ms, 55]; ob.push[ms, 66]; ob.push[ms, 77]; System.out.println["Item popped is " + ob.pop[ms]]; System.out.println["Item popped is " + ob.pop[ms]]; System.out.println["Middle Element is " + ob.findMiddle[ms]]; } } // This code is contributed by Sumit Ghosh |
Python3
|
''' Python3 Program to implement a stack that supports findMiddle[] and deleteMiddle in O[1] time ''' ''' A Doubly Linked List Node ''' class DLLNode: def __init__[self, d]: self.prev = None self.data = d self.next = None ''' Representation of the stack data structure that supports findMiddle[] in O[1] time. The Stack is implemented using Doubly Linked List. It maintains pointer to head node, pointer to middle node and count of nodes ''' class myStack: def __init__[self]: self.head = None self.mid = None self.count = 0 ''' Function to create the stack data structure ''' def createMyStack[]: ms = myStack[] ms.count = 0 return ms ''' Function to push an element to the stack ''' def push[ms, new_data]: ''' allocate DLLNode and put in data ''' new_DLLNode = DLLNode[new_data] ''' Since we are adding at the beginning, prev is always NULL ''' new_DLLNode.prev = None ''' link the old list off the new DLLNode ''' new_DLLNode.next = ms.head ''' Increment count of items in stack ''' ms.count += 1 ''' Change mid pointer in two cases 1] Linked List is empty 2] Number of nodes in linked list is odd ''' if[ms.count == 1]: ms.mid = new_DLLNode else: ms.head.prev = new_DLLNode # Update mid if ms->count is odd if[[ms.count % 2] != 0]: ms.mid = ms.mid.prev ''' move head to point to the new DLLNode ''' ms.head = new_DLLNode ''' Function to pop an element from stack ''' def pop[ms]: ''' Stack underflow ''' if[ms.count == 0]: print["Stack is empty"] return -1 head = ms.head item = head.data ms.head = head.next # If linked list doesn't become empty, # update prev of new head as NULL if[ms.head != None]: ms.head.prev = None ms.count -= 1 # update the mid pointer when # we have even number of elements # in the stack, i,e move down # the mid pointer. if[ms.count % 2 == 0]: ms.mid = ms.mid.next return item # Function for finding middle of the stack def findMiddle[ms]: if[ms.count == 0]: print["Stack is empty now"] return -1 return ms.mid.data # Driver code if __name__ == '__main__': ms = createMyStack[] push[ms, 11] push[ms, 22] push[ms, 33] push[ms, 44] push[ms, 55] push[ms, 66] push[ms, 77] print["Item popped is " + str[pop[ms]]] print["Item popped is " + str[pop[ms]]] print["Middle Element is " + str[findMiddle[ms]]] # This code is contributed by rutvik_56. |
C#
|
/* C# Program to implement a stack that supports findMiddle[] and deleteMiddle in O[1] time */ using System; class GFG { /* A Doubly Linked List Node */ public class DLLNode { public DLLNode prev; public int data; public DLLNode next; public DLLNode[int d] { data = d; } } /* Representation of the stack data structure that supports findMiddle[] in O[1] time. The Stack is implemented using Doubly Linked List. It maintains pointer to head node, pointer to middle node and count of nodes */ public class myStack { public DLLNode head; public DLLNode mid; public int count; } /* Function to create the stack data structure */ myStack createMyStack[] { myStack ms = new myStack[]; ms.count = 0; return ms; } /* Function to push an element to the stack */ void push[myStack ms, int new_data] { /* allocate DLLNode and put in data */ DLLNode new_DLLNode = new DLLNode[new_data]; /* Since we are adding at the beginning, prev is always NULL */ new_DLLNode.prev = null; /* link the old list off the new DLLNode */ new_DLLNode.next = ms.head; /* Increment count of items in stack */ ms.count += 1; /* Change mid pointer in two cases 1] Linked List is empty 2] Number of nodes in linked list is odd */ if [ms.count == 1] { ms.mid = new_DLLNode; } else { ms.head.prev = new_DLLNode; // Update mid if ms->count is odd if [[ms.count % 2] != 0] ms.mid = ms.mid.prev; } /* move head to point to the new DLLNode */ ms.head = new_DLLNode; } /* Function to pop an element from stack */ int pop[myStack ms] { /* Stack underflow */ if [ms.count == 0] { Console.WriteLine["Stack is empty"]; return -1; } DLLNode head = ms.head; int item = head.data; ms.head = head.next; // If linked list doesn't become empty, // update prev of new head as NULL if [ms.head != null] ms.head.prev = null; ms.count -= 1; // update the mid pointer when // we have even number of elements // in the stack, i,e move down // the mid pointer. if [ms.count % 2 == 0] ms.mid = ms.mid.next; return item; } // Function for finding middle of the stack int findMiddle[myStack ms] { if [ms.count == 0] { Console.WriteLine["Stack is empty now"]; return -1; } return ms.mid.data; } // Driver code public static void Main[String[] args] { GFG ob = new GFG[]; myStack ms = ob.createMyStack[]; ob.push[ms, 11]; ob.push[ms, 22]; ob.push[ms, 33]; ob.push[ms, 44]; ob.push[ms, 55]; ob.push[ms, 66]; ob.push[ms, 77]; Console.WriteLine["Item popped is " + ob.pop[ms]]; Console.WriteLine["Item popped is " + ob.pop[ms]]; Console.WriteLine["Middle Element is " + ob.findMiddle[ms]]; } } // This code is contributed // by Arnab Kundu |
Output Item popped is 77 Item popped is 66 Item popped is 55 Middle Element is 33 Deleted Middle Element is 33 Middle Element is 22
This article is contributed by Chandra Prakash. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
Article Tags :
Stack
Practice Tags :
Stack
Read Full Article