560. Subarray Sum Equals K

Medium

https://leetcode.com/problems/subarray-sum-equals-k/

 

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

Note:

1. The length of the array is in range [1, 20,000].
2. The range of numbers in the array is [-1000, 1000] and the range of the integer k is [-1e7, 1e7].

方法一：（参考：https://zhuanlan.zhihu.com/p/36439368）

考点：prefixSum array

维护一个前缀字典 prefix = {}，该字典里的 key 为一个前缀和，相应的 value 为该前缀和出现的次数。令 prefixSum 表示当前位置的前缀和，所以在每个位置我们都得到了以这个位置为结尾的并且和等于 k 的区间的个数，也就是 prefixSum - k 在前缀字典里出现的次数。注意需要设 prefix[0] = 1。

class Solution(object):
    def subarraySum(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """
        cnt = 0
        prefixSum = 0
        prefix = {}
        prefix[0] = 1
        for n in nums:
            prefixSum += n
            cnt += prefix.get(prefixSum - k, 0)
            prefix[prefixSum] = prefix.get(prefixSum, 0) + 1 
        return cnt