25. 735. Asteroid Collision

class Solution {
    public int[] asteroidCollision(int[] asteroids) {
        Stack<Integer> st = new Stack<Integer>();

        for(int asteroid:asteroids){
            //right //empty
            if(st.isEmpty()|| asteroid>0){
                st.push(asteroid);
            }else{
                //right and left (collision possible)
                while(!st.isEmpty() && st.peek()>0){
                    int right = st.peek();

                    //equal
                    if(right == -asteroid){
                        asteroid = 0;
                        st.pop();
                        break;
                    }
                    //less (r < l)
                    else if(right < -asteroid){
                        st.pop();
                    }
                    //r > l
                    else{
                        asteroid = 0;
                        break;
                    }

                }

                if(asteroid !=0){
                    st.push(asteroid);
                }
            }
        }

        int res[] = new int[st.size()];
        for(int i=st.size()-1;i>=0;i--){
            res[i] = st.pop();
        }
        return res;
        
    }
}