![https://leetcode.com/problems/min-stack/]

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.
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Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

Constraints:

  • Methods pop, top and getMin operations will always be called on non-empty stacks.
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class MinStack {
    
    private List<Integer> stack = new ArrayList<>();
    private List<Integer> min = new ArrayList<>();

    /** initialize your data structure here. */
    public MinStack() {
        
    }
    
    public void push(int x) {
        stack.add(x);
        int prev = min.size() > 0 ? min.get(min.size() - 1): Integer.MAX_VALUE;
        min.add(Math.min(prev, x));
    }
    
    public void pop() {
        stack.remove(stack.size() - 1);
        min.remove(min.size() - 1);
    }
    
    public int top() {
        return stack.get(stack.size() - 1);
    }
    
    public int getMin() {
        return min.get(min.size() - 1);
    }
}

/**
 * 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();
 */