Stack Question

Author: Vinay Patel

NeetCode_150_DSA/Stack/Min_Stack_2.java

@@ -0,0 +1,58 @@
+package NeetCode_150_DSA.Stack;
+
+import java.util.Stack;
+
+class MinStack {
+ private Stack<Integer> s; // stack
+ private Stack<Integer> miniS; // minStack
+
+ public MinStack() {
+ s = new Stack<>();
+ miniS = new Stack<>();
+ }
+
+ public void push(int value) {
+ s.push(value);
+
+ if (miniS.isEmpty() || value <= miniS.peek()) {
+ miniS.push(value);
+ }
+ }
+
+ public void pop() {
+ if (!s.isEmpty()) {
+ int popped = s.pop();
+
+ if (!miniS.isEmpty() && popped == miniS.peek()) {
+ miniS.pop();
+ }
+ }
+ }
+
+ public int top() {
+ if (!s.isEmpty()) {
+ return s.peek();
+ }
+ return -1;
+ }
+
+ public int getMin() {
+ if (!miniS.isEmpty()) {
+ return miniS.peek();
+ }
+ return -1;
+ }
+
+}
+
+public class Min_Stack_2 {
+ public static void main(String[] args) {
+ MinStack obj = new MinStack();
+ int value;
+ obj.push(value);
+ obj.pop();
+ int param_3 = obj.top();
+ int param_4 = obj.getMin();
+ }
+
+}

NeetCode_150_DSA/Stack/Valid_Parentheses_1.java

@@ -0,0 +1,53 @@
+package NeetCode_150_DSA.Stack;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Stack;
+
+public class Valid_Parentheses_1 {
+
+ public static void main(String[] args) {
+
+ }
+ public boolean isValid(String s) {
+ // Brute Force
+ /*
+ Stack<Character> stack = new Stack<>();
+
+ for(char c:s.toCharArray()){
+ if(c=='(' || c=='{' || c=='['){
+ stack.push(c);
+ }else if(c==')' && !stack.isEmpty() && stack.peek()=='('){
+ stack.pop();
+ }else if(c=='}' && !stack.isEmpty() && stack.peek()=='{'){
+ stack.pop();
+ }else if(c==']' && !stack.isEmpty() && stack.peek()=='['){
+ stack.pop();
+ }else{
+ return false;
+ }
+ }
+ return stack.isEmpty();
+ */
+
+ // We can eliminate the need for the separate if-else statements by using a map to store the mappings of opening and closing parentheses
+ Stack<Character> stack = new Stack<>();
+ Map<Character, Character> mappings = new HashMap<>();
+
+ mappings.put(')','(');
+ mappings.put('}','{');
+ mappings.put(']','[');
+
+ for(char c:s.toCharArray()){
+ if(mappings.containsKey(c)){
+ char topEle = stack.isEmpty() ? '#' : stack.pop();
+ if(topEle != mappings.get(c)){
+ return false;
+ }
+ }else{
+ stack.push(c);
+ }
+ }
+ return stack.isEmpty();
+ }
+}