Lc735

class Solution(object):
    def asteroidCollision(self, asteroids):
        """
        :type asteroids: List[int]
        :rtype: List[int]
        main idea: loop through each asteroid and compare it with stack top. discuss especilaly when stack top is positive and current asteroid is negtive
        tricky part: if stack top is positive and current asteroid is neg, and abs(top) < abs(cur), stack pop off top asteroid, current asteroid will need to compare with next stack top until top pop condition not exist.
        """
        # time O(N) space O(N)
        st = []
        for a in asteroids:
            #  case1: top is -, a > 0; case2: top + a - ; case 3: top + a+ or top - a-  
            while st and a<0<st[-1] :
                #case 1 abs(a) > top ; case2 abs(a) < top ;case3: abs(a) == top 
                if abs(st[-1]) == abs(a):
                    st.pop()
                elif abs(st[-1]) < abs(a):
                    st.pop()
                    continue
                break
            else:
                st.append(a)
        return st