maximum-sum-of-3-non-overlapping-subarrays.py

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

# In a given array nums of positive integers, find three non-overlapping subarrays with maximum sum.
#
# Each subarray will be of size k, and we want to maximize the sum of all 3*k entries.
#
# Return the result as a list of indices representing the starting position of each interval (0-indexed).
# If there are multiple answers, return the lexicographically smallest one.
#
# Example:
# Input: [1,2,1,2,6,7,5,1], 2
# Output: [0, 3, 5]
 #
# Explanation: Subarrays [1, 2], [2, 6], [7, 5] correspond to the starting indices [0, 3, 5].
# We could have also taken [2, 1], but an answer of [1, 3, 5] would be lexicographically larger.
#
# Note:
# - nums.length will be between 1 and 20000.
# - nums[i] will be between 1 and 65535.
# - k will be between 1 and floor(nums.length / 3).

class Solution(object):
    def maxSumOfThreeSubarrays(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: List[int]
        """
        n = len(nums)
        accu = [0]
        for num in nums:
            accu.append(accu[-1]+num)

        left_pos = [0] * n
        total = accu[k]-accu[0]
        for i in xrange(k, n):
            if accu[i+1]-accu[i+1-k] > total:
                left_pos[i] = i+1-k
                total = accu[i+1]-accu[i+1-k]
            else:
                left_pos[i] = left_pos[i-1]

        right_pos = [n-k] * n
        total = accu[n]-accu[n-k]
        for i in reversed(xrange(n-k)):
            if accu[i+k]-accu[i] > total:
                right_pos[i] = i;
                total = accu[i+k]-accu[i]
            else:
                right_pos[i] = right_pos[i+1]

        result, max_sum = [], 0
        for i in xrange(k, n-2*k+1):
            left, right = left_pos[i-1], right_pos[i+k]
            total = (accu[i+k]-accu[i]) + \
                    (accu[left+k]-accu[left]) + \
                    (accu[right+k]-accu[right])
            if total > max_sum:
                max_sum = total
                result = [left, i, right]
        return result