chore(Java): add finding min value on stack (#622)

algorithms/Java/README.md

@@ -71,6 +71,7 @@
 - [The Stock Span Problem](stacks/the-stock-span-problem.java)
 - [Celebrity Problem](stacks/celebrity-problem.java)
 - [Sliding Window Maximum](stacks/sliding-window-maximum.java)
+- [Min Stack](stacks/Min-Stack.java)
 
 ## Strings
 

algorithms/Java/stacks/Min-Stack.java

@@ -0,0 +1,83 @@
+// Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+
+// Implement the MinStack class:
+
+// MinStack() initializes the stack object.
+// void push(int val) pushes the element val onto the stack.
+// void pop() removes the element on the top of the stack.
+// int top() gets the top element of the stack.
+// int getMin() retrieves the minimum element in the stack.
+ 
+
+// Java program to implement a stack that supports
+// getMinimum() in O(1) time and O(1) extra space.
+import java.util.*;
+
+// A user defined stack that supports getMin() in
+// addition to push() and pop()
+class MinStack {
+    long min = 0;
+    Stack<Long> stack = new Stack<>();
+    
+    public void push(int x) {
+        // only push the old minimum value when the current 
+        // minimum value changes after pushing the new value x
+        if(stack.size() == 0){
+            stack.push((long) x);
+            min = x;
+        }
+        else if(x < min){          
+            stack.push((long) 2*x - min);
+            min=x;
+        }else
+        stack.push((long) x);
+    }
+
+    public void pop() {
+        // if pop operation could result in the changing of the current minimum value, 
+        // pop twice and change the current minimum value to the last minimum value.
+        if(stack.peek() < min) {
+            min = 2*min - stack.pop();
+        }else
+            stack.pop();
+    }
+
+    public int top() {
+        if (stack.peek() < min)
+            return (int) min;
+        else
+            return (int) (long) stack.peek();
+    }
+
+    public int getMin() {
+        return (int) min;
+    }
+}
+
+// Driver Code
+public class Main
+{
+  public static void main(String[] args)
+  {
+    MinStack s = new MinStack();
+    s.push(3);
+    s.push(5);
+    System.out.println("Min element present is: "+ s.getMin());
+    s.push(2);
+    s.push(1);
+    System.out.println("Min element present is: "+ s.getMin());
+    s.pop();
+    System.out.println("Min element present is: "+ s.getMin());
+    s.pop();
+    System.out.println("Top element present is: " + s.top());
+  }
+}
+
+//TC: O(1)
+//SC: O(1)
+// Output
+// Min element present is: 3
+// Min element present is: 1
+// Min element present is: 2
+// Top element present is: 5
+