Description

Given an array of integers and an integer k, you need to find the total number of continuous subarrays whose sum equals to k.

Example 1:
Input:nums = [1,1,1], k = 2
Output: 2

Solution

1.build累积数组和
2.遍历，过程中 先check sum in dict, then add value for dict （这样可以去重复

class Solution:
    def subarraySum(self, nums: List[int], k: int) -> int:
        cnt =  0
        if len(nums)==0:
            return cnt
        max = [nums[0]]
        
        for i in range(1,len(nums)):
            max.append(nums[i]+max[-1])
        d = {0:1} #相当于从开始1st ele的sum满足K的sum
        for i in range(len(nums)):
            if max[i]-k in d:
                cnt += d[max[i]-k]
            if max[i] in d:
                d[max[i]]+=1
            else:
                d[max[i]]=1
        return cnt