class Solution:
def findTargetSumWays(self, nums: List[int], target: int) -> int:
s = sum(nums)
diff = s - target
if diff < 0 or diff % 2 != 0: #无法构造
return 0
neg = diff // 2
n = len(nums)
dp = [[0] * (neg+1) for i in range(n+1)]
for i in range(n+1): #关键,base case
dp[i][0] = 1
for i in range(1, n+1):
for j in range(neg+1):
if j-nums[i-1] >= 0:
dp[i][j] = dp[i-1][j] + dp[i-1][j-nums[i-1]]
else:
dp[i][j] = dp[i-1][j]
return dp[n][neg]