[LeetCode] Problem 155 - Min Stack

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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.

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.

Code

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// O(n) runtime, O(n) space – Minor space optimization
class MinStack {

    private Stack<Integer> stack;
    private Stack<Integer> min;

    public MinStack() {
        stack = new Stack<>();
        min = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);

        if (min.isEmpty() || x <= min.peek())
            min.push(x);
    }

    public void pop() {
        int x = stack.pop();

        if (x == min.peek())
            min.pop();
    }

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

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