new question added

Author: arorarahul

README.md

@@ -159,6 +159,7 @@ TODO:
 - [Implement a stack using arrays & linked-list](/stacks-and-queues/question1.c)
 - [Implement a queue using circular array](/stacks-and-queues/question2.c)
 - [Implement a queue using two stacks](/stacks-and-queues/question3.c)
+- [](/stacks-and-queues/question4.c)
 
 
 ## Some important concepts to solve algos better
@@ -171,6 +172,8 @@ TODO:
 - Linear hashing is (h(k) + i)modm (where m is the size of the hash table, h(k) is the hash function that takes the key k and returns a value i is the parameter that is incremented to get different values)
 - Subarrays of an array are always contiguous whereas subsequence may not be contiguous
 - There are only two ways to make any data structure one is to use an array (where size is fixed and memory is contiguous) OR you can use the heap memory (structures and linked list). So Array and linked list or combo of these two are used to implement any data structure. In most cases linked list takes more time to do operations if data structure is implemented using it. But advantage is dynamic memory allocation
+- For INFIX to POSTFIX conversion, data structure used is stack. In stack all the operators are stored. For evaluation of POSTFIX stack is used to store operands
+- Evaluating and expression = convert from INFIX to postfix --> Evaluate POSTFIX. Time complexity: O(n)
 
 # Topic1: Introduction
 

nextquestions.md

@@ -26,4 +26,6 @@
 - how to accurately find the size of the hashTable
 - BST question5 hashing to be done
 - hashing question7 method3 to be done using BST
-- question4 hashing to be done
+- question4 hashing to be done
+- stacks and queues better implementation to be done
+- stacks and queues INFIX to POSTFIX & evaluating POSTFIX

stacks-and-queues/question3.c

@@ -5,4 +5,75 @@ If n elements are to be inserted on to a queue implemented using two stacks and
           Push Pop
 Best case m+n  2m
 Work Case 2n   n+m
-*/
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#define MAX 50
+
+void push(int arr[],int data,int *top){
+	if(*top == MAX-1){
+		printf("overflow\n");
+		return;
+	}
+	arr[++*top]=data;
+}
+
+void insert(int arr[],int data, int *top){
+	push(arr,data,top);
+}
+
+int pop(int arr[], int *top){
+	int data = arr[*top];
+	*top--;
+	return data;
+}
+
+int delete(int arr2[], int arr1[],int *top2, int *top1){
+	int elm;
+	if(*top2==-1){
+		if(*top1==-1){
+			return -1; //this can be an error code
+		}
+	}else{
+		while(*top2 >= 0){
+			int data = pop(arr2, *top2);
+			*top2--;
+			push(arr1,data,*top1);
+		}
+		elm = pop(arr1,top1);
+		return elm;
+	}
+}
+
+int main(){
+	int stack1[MAX],stack2[MAX];
+	int top1 = -1, top2 = -1;
+	int step, elm, result;
+
+	while(1){
+		printf("1. PUSH element\n");
+		printf("2. POP element\n");
+		printf("3.EXIT \n");
+		scanf("%d",&step);
+
+		switch(step){
+			case 1: printf("enter element to be pushed\n");
+				scanf("%d",&elm);
+				insert(stack2,elm, &top2);
+				break;
+			case 2: result = delete(stack2,stack1,&top2,&top1);
+				if(result == -1){
+					printf("already empty\n");
+				}else{
+					printf("%d was popped\n", result);
+				}
+
+				break;
+			case 3: exit(1);
+				break;
+		}
+	}
+
+	return 0;
+}

stacks-and-queues/question4.c

@@ -0,0 +1,3 @@
+/*
+
+*/