Update 0494-target-sum.py (runtime and memory > 80%)

[purnawirman]

File(s) Modified: 0494-target-sum.py
Language(s) Used: _python
Submission URL: https://leetcode.com/problems/target-sum/submissions/905306462/

optimize more by:

1. using only 2 rows of dp
2. iterate only on non zero previous row dp

[raul-sauco]

Same idea but using a Counter makes it a bit more readable, in my opinion, and the performance is similar.

Runtime 304 ms Beats 87.41%
Memory 14.1 MB Beats 88.17%

https://leetcode.com/problems/target-sum/submissions/842843238/

class Solution:
    def findTargetSumWays(self, nums: List[int], target: int) -> int:
        dp = Counter([0])
        for num in nums:
            next = Counter()
            for key in dp.keys():
                next[key - num] += dp[key]
                next[key + num] += dp[key]
            dp = next
        return dp[target]