continuous-subarray-sum.py

# Time:  O(n)
# Space: O(k)

# Given a list of non-negative numbers and a target integer k,
# write a function to check if the array has a continuous subarray
# of size at least 2 that sums up to the multiple of k, that is,
# sums up to n*k where n is also an integer.
#
# Example 1:
# Input: [23, 2, 4, 6, 7],  k=6
# Output: True
# Explanation: Because [2, 4] is a continuous subarray of size 2 and sums up to 6.
# Example 2:
# Input: [23, 2, 6, 4, 7],  k=6
# Output: True
# Explanation: Because [23, 2, 6, 4, 7] is an continuous subarray of size 5 and sums up to 42.
# Note:
# The length of the array won't exceed 10,000.
# You may assume the sum of all the numbers is in the range of a signed 32-bit integer.

class Solution(object):
    def checkSubarraySum(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: bool
        """
        count = 0
        lookup = {0: -1}
        for i, num in enumerate(nums):
            count += num
            if k:
                count %= k
            if count in lookup:
                if i - lookup[count] > 1:
                    return True
            else:
                lookup[count] = i

        return False