Question

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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.

You must implement a solution with O(1) time complexity for each function.

Analysis

辅助 stack （存最小值）。

I covered this question in another post [Question] Min Stack.

Code

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class MinStack {

	Stack<Integer> stack = new Stack<Integer>();
	Stack<Integer> min = new Stack<Integer>();

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

	public void pop() {
		if (stack.isEmpty()) {
			return;
		}
		int topNum = stack.pop();
		if (topNum == min.peek()) {
			min.pop();
		}
	}

	public int top() {
		if (stack.isEmpty()) {
			return 0;
		}
		return stack.peek();
	}

	public int getMin() {
		if (min.isEmpty()) {
			return 0;
		}
		return min.peek();
	}
}