MinStack.java

public class MinStack {
    
    private List<Integer> data;
    private int top_idx;

    /** initialize your data structure here. */
    public MinStack() {
        data = new ArrayList<Integer>();
        top_idx = -1;
    }
    
    public void push(int x) {
        top_idx++;
        data.add(top_idx, x);
    }
    
    public void pop() {
        data.remove(top_idx);
        top_idx--;
    }
    
    public int top() {
        return data.get(top_idx);
    }
    
    public int getMin() {
        if(top_idx < 0) throw new RuntimeException("no elements in stack");
        int min = data.get(0);
        for(int i=0; i<= top_idx; i++) {
            min = Math.min(min, data.get(i));
        }
        return min;
    }
}

/**
 * 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();
 */