TheAlgorithms-Ruby/data_structures/arrays/maximum_subarray.rb

#Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum.
#A subarray is a contiguous part of an array.

#Example 1:
#Input: nums = [-2,1,-3,4,-1,2,1,-5,4]
#Output: 6
#Explanation: [4,-1,2,1] has the largest sum = 6.

#Example 2:
#Input: nums = [1]
#Output: 1

#Example 3:
#Input: nums = [5,4,-1,7,8]
#Output: 23

#Constraints:
#1 <= nums.length <= 3 * 104
#-105 <= nums[i] <= 105




#Dynamic Programming Approach (Kadane's Algorithm) - O(n) Time / O(1) Space
#Init max_sum as first element
#Return first element if the array length is 1
#Init current_sum as 0
#Iterate through the array:
#if current_sum < 0, then reset it to 0 (to eliminate any negative prefixes)
#current_sum += num
#max_sum = current_sum if current_sum is greater than max_sum
#Return max_sum   


# @param {Integer[]} nums
# @return {Integer}
def max_sub_array(nums)
    #initialize max sum to first number
    max_sum = nums[0]
    
    #return first number if array length is 1 
    return max_sum if nums.length == 1
    
    #init current sum to 0
    current_sum = 0
    
    #iterate through array, reset current_sum to 0 if it ever goes below 0, track max_sum with highest current_sum
    nums.each do |num|
      current_sum = 0 if current_sum < 0
      current_sum += num
      max_sum = [max_sum, current_sum].max
    end
    
    #return answer
    max_sum
end