560. 和为K的子数组

给定一个整数数组和一个整数 k，你需要找到该数组中和为 k 的连续的子数组的个数。

示例  :

输入:nums = [1,1,1], k = 2
输出: 2 ,  [1,1] 与 [1,1] 为两种不同的情况。

 思路1：字典记录前缀和

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class Solution:     def subarraySum(self, nums: List[int], k: int) -> int:         num_times = collections.defaultdict(int)         num_times[0] = 1         cur_sum = 0         res = 0         for i in range(len(nums)):             cur_sum += nums[i]              if cur_sum - k in num_times:                  res += num_times[cur_sum - k]             num_times[cur_sum] += 1         return res

思路2：动态规划（OJ超时）

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class Solution: #超时     def subarraySum(self, nums: List[int], k: int) -> int:         N = len(nums)         dp = [0]*N         dp[0] = 0 if nums[0]!=k else 1         for i in range(1,N):             temp = 0             flag = 0             for j in range(i,-1,-1):                 temp+=nums[j]                 if temp == k:                     flag += 1             dp[i] = dp[i-1]+flag         return dp[-1]