Maximum Subarray Sum

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.

O(N^3) Solution

• Brute force
• We will check for all the subarray and find the maximum sum.

•   for(i=0,n)
        for (j=i,n)
            sum=0
            for(k=i,j)
                sum+=arr[k]
                check for max subarray sum
  

O(N^2) Solution

• We are running 3rd loop from i to j, we can do this by using only 2 loops.
• I n the beginning of 2nd loop set sum = 0 and in 2nd loop keep adding elements to sum and update the max sum.

•   for(i=0,n)
        sum=0;
        for (j=i,n)
            sum+= arr[i];
            maxS=max(maxS,sum);
  

O(N) time constant space(DP)/Kadane’s algorithm

• We maintain a maximum sum and current sum(if element itself is max)
• for each i=(0,n)
  • add current element to current sum
  • mx = max(current sum , mx);
  • current sum will be max(0,current sum);
• finally we return mx

Code

// codestudio
#include <bits/stdc++.h>
long long maxSubarraySum(int arr[], int n)
{
    long long maxSum = 0;
    long long currSum = 0;
    for (int i = 0; i < n; i++) {
        currSum += arr[i];
        maxSum = max(maxSum, currSum); // end = i
        currSum = max(0ll, currSum); // start = i+1
    }
    return maxSum;
}

To get subarray

• While updating current sum, set start pointer to i+1.
• While updating max sum, set end pointer to i.
• Our subarray is from start to end.