155. Min Stack

/*
https://leetcode.com/problems/min-stack/description/
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.
*/

class MinStack {
        static class StackElement {
            private int value;
            private int minBelow;

            public StackElement() {
            }

            public StackElement(int value, int minBelow) {
                this.value = value;
                this.minBelow = minBelow;
            }

            public int getValue() {
                return value;
            }

            public int getMinBelow() {
                return minBelow;
            }
        }

        private StackElement[] stack;
        private int topIdx;
        private int minimum = 999;
        private static final int initialLength = 10000;


        /**
         * initialize your data structure here.
         */
        public MinStack() {
            this.stack = new StackElement[initialLength];
            this.topIdx = -1;
        }

        public void push(int x) {
            if (topIdx == -1) {
                minimum = x;
            }

            stack[++topIdx] = new StackElement(x, x < minimum ? x : minimum);
            minimum = minimum < x ? minimum : x;
        }

        public void pop() {
            topIdx--;
            if (topIdx > -1) {
                minimum = stack[topIdx].getMinBelow();
            }
        }

        public int top() {
            return stack[topIdx].getValue();
        }

        public int getMin() {
            return minimum;
        }
    }