Merge pull request Sahiljawale#62 from viveksondhiya/patch-4

Stack implementation and Imp questions

Java/Stack concept java

@@ -0,0 +1,140 @@
+ //Implementing stack (from scrach)using Arraylist
+ static class Stack{
+ static ArrayList<Integer> s=new ArrayList<>();
+
+ public static boolean isEmpty(){
+ return s.size()==0;
+ }
+ public static void push(int data){
+ s.add(data);
+ }
+ static int pop(){
+ if(s.isEmpty()){
+ return -1;
+ }
+ int top=s.get(s.size()-1);
+ s.remove(s.size()-1);
+ return top;
+ }
+ public int peek(){
+ if(s.isEmpty()){
+ return -1;
+ }
+ return s.get(s.size()-1);
+ }
+
+ }
+
+ //Implenenting stack using linkedlist
+
+ static class Node{
+ int data;
+ Node next;
+ Node(int data){
+ this.data=data;
+ this.next=null;
+ }
+ }
+ static class Stack{
+ static Node head=null;
+
+ public static boolean isEmpty(){
+ return head==null;
+ }
+
+ static void push(int data){
+ Node newNode=new Node(data);
+
+ if(isEmpty()){
+ head=newNode;
+ return;
+ }
+ newNode.next=head;
+ head=newNode;
+ }
+
+ static int pop(){
+ if(isEmpty()){
+ return -1;
+ }
+ int top=head.data;
+ head=head.next;
+ return top;
+ }
+
+ static int peek(){
+ if(isEmpty()){
+ return -1;
+ }
+ return head.data;
+ }
+ }
+
+ //Dublicate parenthesis
+ public static boolean isDublicate(String str){
+ Stack<Character> s=new Stack<>();
+ for(int i=0;i<str.length();i++){
+ char ch=str.charAt(i);
+ if(ch==')'){
+ int count=0;
+ while(s.peek()!='('){
+ s.pop();
+ count++;
+ }
+ if(count<1){
+ return true;
+ }else{
+ s.pop();
+ }
+ }else{
+ s.push(ch);
+ }
+ }
+ return false;
+ }
+ //Push bottom
+ public static void pushatBottom(Stack<Integer> s ,int data){
+
+ if(s.isEmpty()){
+ s.push(data);
+ return;
+ }
+
+ int top=s.pop();
+ pushatBottom(s, data);
+ s.push(top);
+ }
+ //reverse stack
+ public static void pushBottom(Stack<Integer> s, int data) {
+ if (s.isEmpty()) {
+ s.push(data);
+ return;
+ }
+ int top = s.pop();
+ pushBottom(s, data);
+ s.push(top);
+ }
+
+ public static void reverseStack(Stack<Integer> s) {
+ if (s.isEmpty()) {
+ return;
+ }
+ int top = s.pop();
+ reverseStack(s);
+ pushBottom(s, top);
+ }
+ //reverse string using stack
+ public static String reverseString(String str){
+ Stack<Character> s=new Stack<>();
+ int idx=0;
+ while(idx<str.length()){
+ s.push(str.charAt(idx));
+ idx++;
+ }
+ StringBuilder ans=new StringBuilder(" ");
+ while(!s.isEmpty()){
+ char curr=s.pop();
+ ans.append(curr);
+ }
+ return ans.toString();
+ }