Added StackUtils.java - This utility provides commonly used operations related to stacks

Author: mohit-gogitter

src/main/java/com/thealgorithms/stacks/StackUtils.java

@@ -0,0 +1,142 @@
+import java.util.Stack;
+
+/** 
+ * This utility provides commonly used operations related to stacks, which can be beneficial for solving various problems such as expression evaluations, balanced parentheses, or maintaining a history of operations
+ * MinStack: This custom stack keeps track of the minimum element so that getMin() can return the minimum in O(1) time.
+ * Next Greater Element: For each element in an array, the utility finds the next greater element on the right side using a stack.
+ * Balanced Parentheses Checker: It checks if an expression has matching and balanced parentheses.
+ * Reverse a Stack: This utility reverses the elements in a stack using recursion.
+ * @author Mohit Singh
+ * @author <a href="https://github.com/mohit-gogitter">mohit-gogitter<a>
+ */
+
+public class Main {
+
+    // 1. MinStack - Stack supporting push, pop, and retrieving minimum in O(1) time
+    static class MinStack {
+        private Stack<Integer> mainStack;
+        private Stack<Integer> minStack;
+
+        public MinStack() {
+            mainStack = new Stack<>();
+            minStack = new Stack<>();
+        }
+
+        public void push(int value) {
+            mainStack.push(value);
+            if (minStack.isEmpty() || value <= minStack.peek()) {
+                minStack.push(value);
+            }
+        }
+
+        public void pop() {
+            if (!mainStack.isEmpty()) {
+                int poppedValue = mainStack.pop();
+                if (poppedValue == minStack.peek()) {
+                    minStack.pop();
+                }
+            }
+        }
+
+        public int getMin() {
+            return minStack.isEmpty() ? Integer.MAX_VALUE : minStack.peek();
+        }
+
+        public int top() {
+            return mainStack.isEmpty() ? -1 : mainStack.peek();
+        }
+
+        public boolean isEmpty() {
+            return mainStack.isEmpty();
+        }
+    }
+
+    // 2. Next Greater Element for each element in an array
+    public static int[] nextGreaterElement(int[] nums) {
+        Stack<Integer> stack = new Stack<>();
+        int[] result = new int[nums.length];
+
+        for (int i = nums.length - 1; i >= 0; i--) {
+            while (!stack.isEmpty() && stack.peek() <= nums[i]) {
+                stack.pop();
+            }
+            result[i] = stack.isEmpty() ? -1 : stack.peek();
+            stack.push(nums[i]);
+        }
+        return result;
+    }
+
+    // 3. Balanced Parentheses Checker
+    public static boolean isBalanced(String expression) {
+        Stack<Character> stack = new Stack<>();
+        for (char c : expression.toCharArray()) {
+            if (c == '(' || c == '{' || c == '[') {
+                stack.push(c);
+            } else if (c == ')' || c == '}' || c == ']') {
+                if (stack.isEmpty())
+                    return false;
+                char open = stack.pop();
+                if (!isMatchingPair(open, c))
+                    return false;
+            }
+        }
+        return stack.isEmpty();
+    }
+
+    private static boolean isMatchingPair(char open, char close) {
+        return (open == '(' && close == ')') ||
+                (open == '{' && close == '}') ||
+                (open == '[' && close == ']');
+    }
+
+    // 4. Reverse a Stack
+    public static <T> void reverseStack(Stack<T> stack) {
+        if (stack.isEmpty())
+            return;
+        T temp = stack.pop();
+        reverseStack(stack);
+        insertAtBottom(stack, temp);
+    }
+
+    private static <T> void insertAtBottom(Stack<T> stack, T value) {
+        if (stack.isEmpty()) {
+            stack.push(value);
+        } else {
+            T temp = stack.pop();
+            insertAtBottom(stack, value);
+            stack.push(temp);
+        }
+    }
+
+    public static void main(String[] args) {
+        // Example usage of MinStack
+        MinStack minStack = new MinStack();
+        minStack.push(3);
+        minStack.push(5);
+        minStack.push(2);
+        minStack.push(1);
+        System.out.println("Current Min: " + minStack.getMin()); // Output: 1
+        minStack.pop();
+        System.out.println("Current Min after pop: " + minStack.getMin()); // Output: 2
+
+        // Example usage of nextGreaterElement
+        int[] nums = { 4, 5, 2, 10, 8 };
+        int[] nextGreater = nextGreaterElement(nums);
+        System.out.println("Next Greater Elements: ");
+        for (int val : nextGreater) {
+            System.out.print(val + " ");
+        } // Output: 5 10 10 -1 -1
+
+        // Example usage of isBalanced
+        String expr = "{[()]}";
+        System.out.println("\nIs the expression balanced? " + isBalanced(expr)); // Output: true
+
+        // Example usage of reverseStack
+        Stack<Integer> stack = new Stack<>();
+        stack.push(1);
+        stack.push(2);
+        stack.push(3);
+        reverseStack(stack);
+        System.out.println("Reversed Stack: " + stack); // Output: [3, 2, 1]
+    }
+}