MinStack.java

/*
# Problem Statement: Implement functions of min stack
---------------------------------------------------------------
# Approach :
- use variations of push and pop
- in push operation if the value is less than then minimum push 2*min - val
- in pop if val<min change the min value and pop
---------------------------------------------------------------
*/

class MinStack {
    Stack<Long> st = new Stack<Long>();
    Long mini;
    public MinStack() {
        mini = Long.MAX_VALUE;
    }
    
    public void push(int val) {
        Long value = Long.valueOf(val);
        if(st.isEmpty()) {
            mini = value;
            st.push(value);
        } else {
            if(value < mini) {
                st.push(2*value - mini);
                mini = value;
            } else {
                st.push(value);
            }
        }
    }
    
    public void pop() {
        if(st.isEmpty()) return ;
        Long value = st.pop();
        if(value<mini) {
            mini = 2*mini - value;
        }
    }
    
    public int top() {
        Long value = st.peek();
        if(value < mini) {
            return mini.intValue();
        }
        return value.intValue();
    }
    
    public int getMin() {
        return mini.intValue();
    }
}