138. Subarray Sum

題目

https://www.lintcode.com/problem/subarray-sum/description?_from=ladder&&fromId=2

實現

1. 定義 prefix_sum_hash 用於存放 { prefix_sum : index }
2. 遍歷整個數組，每次計算 prefix_sum[index]，判斷是否 prefix_sum[index] 是否存在於 hash 裏
   1. 如果存在那麼返回 prefix_sum_hash[prefix_sum] - 1, index
   2. 如果不存在那麼 prefix_sum_hash[prefix_sum] = i
3. 最終返回 -1, -1

代碼

class Solution:
    """
    @param nums: A list of integers
    @return: A list of integers includes the index of the first number and the index of the last number
    """
    def subarraySum(self, nums):
        if nums is None:
            return -1, -1

        prefix_hash = { 0: -1 }
        prefix_sum = 0

        for i, num in enumerate(nums):
            prefix_sum += num

            if prefix_sum in prefix_hash:
                return prefix_hash[prefix_sum] + 1, i

            prefix_hash[prefix_sum] = i

        return -1, -1