Stacks and Queues

Author: Turingfly

src/chapter03StacksAndQueues/ImplementQueueUsingStacks.java

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+package chapter03StacksAndQueues;
+
+import java.util.Stack;
+
+/**
+ * 
+ * @author chengfeili 
+ * Jun 16, 2017 11:49:18 AM
+ * 
+ *         Problem:
+ *
+ *         Solution:
+ *
+ */
+public class ImplementQueueUsingStacks {
+	Stack<Integer> input = new Stack<>();
+	Stack<Integer> output = new Stack<>();
+
+	public void push(int x) {
+		input.push(x);
+	}
+
+	public void pop() {
+		shift();
+		output.pop();
+	}
+
+	public int peek() {
+		shift();
+		return output.peek();
+	}
+
+	public boolean empty() {
+		return input.isEmpty() && output.isEmpty();
+	}
+
+	public void shift() {
+		if (output.isEmpty()) {
+			while (!input.isEmpty())
+				output.push(input.pop());
+		}
+	}
+}

src/chapter03StacksAndQueues/MinStack.java

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+package chapter03StacksAndQueues;
+
+import java.util.Stack;
+
+/**
+ * 
+ * @author chengfeili 
+ * Jun 16, 2017 11:07:40 AM
+ * 
+ *         Problem: 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.
+ *
+ *         Solution: Use an additional stack which keeps track of the mins.
+ *
+ */
+public class MinStack extends Stack<Integer> {
+	private Stack<Integer> stack2 = new Stack<>();
+
+	public void push(int val) {
+		// must be <=
+		if (val <= min()) {
+			stack2.push(val);
+		}
+		super.push(val);
+	}
+
+	public Integer pop() {
+		int val = super.pop();
+		if (val == min()) {
+			stack2.pop();
+		}
+		return val;
+	}
+
+	public Integer min() {
+		if (stack2.isEmpty()) {
+			return Integer.MAX_VALUE;
+		} else {
+			return stack2.peek();
+		}
+	}
+
+	public static void main(String[] args) {
+		MinStack min = new MinStack();
+		min.push(3);
+		min.push(4);
+		min.push(2);
+		System.out.println(min.min());
+	}
+}