lc155.java

// Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
//
// push(x) -- Push element x onto stack.
// pop() -- Removes the element on top of the stack.
// top() -- Get the top element.
// getMin() -- Retrieve the minimum element in the stack.
// Example:
// MinStack minStack = new MinStack();
// minStack.push(-2);
// minStack.push(0);
// minStack.push(-3);
// minStack.getMin();   --> Returns -3.
// minStack.pop();
// minStack.top();      --> Returns 0.
// minStack.getMin();   --> Returns -2.

public class MinStack {
    Stack<Integer> stack;
    Stack<Integer> minStack;
    /** initialize your data structure here. */
    public MinStack() {
      stack = new Stack<Integer>();
      minStack = new Stack<Integer>();
    }

    public void push(int x) {
      stack.push(x);
      if (minStack.isEmpty() || minStack.peek() >= x)
        minStack.push(x);
    }

    public void pop() {
      int value = stack.pop();
      if (value == minStack.peek())
        minStack.pop();
    }

    public int top() {
      return stack.peek();
    }

    public int getMin() {
      return minStack.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */