MinStack done

Author: rampatra

src/main/java/com/ctci/stacksandqueues/StackMin.java

@@ -0,0 +1,63 @@
+package com.ctci.stacksandqueues;
+
+import com.sun.tools.javac.util.Assert;
+
+import java.util.Stack;
+
+/**
+ * How would you design a stack which, in addition to push and pop, has a function min
+ * which returns the minimum element? Push, pop and min should all operate in 0(1) time.
+ * 
+ * @author rampatra
+ * @since 2019-02-04
+ */
+public class StackMin {
+
+    // the main stack to do push, pop, and min operations
+    private static Stack<Integer> stack = new Stack<>();
+    // another stack to store the mins (needed to make min() call O(1))
+    private static Stack<Integer> minStack = new Stack<>();
+
+    private static int push(int item) {
+        minPush(item);
+        return stack.push(item);
+    }
+
+    private static int pop() {
+        minPop(stack.peek());
+        return stack.pop();
+    }
+
+    private static int min() {
+        return minStack.peek();
+    }
+
+    private static void minPush(int item) {
+        if (minStack.empty() || item <= minStack.peek()) {
+            minStack.push(item);
+        }
+    }
+
+    private static void minPop(int item) {
+        if (item == minStack.peek()) {
+            minStack.pop();
+        }
+    }
+
+    public static void main(String[] args) {
+        push(2);
+        push(5);
+        push(1);
+        push(1);
+        push(6);
+        push(8);
+        Assert.check(min() == 1);
+        pop();
+        pop();
+        pop();
+        Assert.check(min() == 1);
+        pop();
+        Assert.check(min() == 2);
+        
+    }
+}