LeetCode_155_Min_Stack.java

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 val=st.pop();
        if(val<mini){
            mini=2*mini-val;
        }
    }
    
    public int top() {
        Long val=st.peek();
        if(val<mini){
            return mini.intValue();
        }
        return val.intValue();
    }
    
    public int getMin() {
        return mini.intValue();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */