kadane_algo.cpp

/* 
	Kadane's Algorithm is used to solve the maximum subarray sum problem.

	The maximum subarray problem is the task of finding the largest 
	possible sum of a contiguous subarray.

	We make three cases in this algorithms.

	1 - all the array elements are postive - then complete array 
	sum is the largest subarray sum.

	2 - all the array elemets are negative - then smallest elements 
	is the largest subarray sum.

	3 - elements are mix of positive and negative numbers - then we keep adding 
	elements in the variable until the variable is positive. We keep track of the
	variable value after each addition, so that we know the maximum value it rose to,
	that maximum value is our answer. 
*/

#include <bits/stdc++.h>
#define ll long long
using namespace std;

main() 
{
	ios_base::sync_with_stdio(false);
    		cin.tie(NULL);
	
	ll int n,maxs;

	// no of elements in the array
	cin>>n;
				  
	ll int arr[n],i,sum=0,pos=0,neg=0, maxi = INT_MIN;
	    
	for(i=0;i<n;i++)
	{
	    // array input
	    cin>>arr[i];
		
	    // complete sum of the array 
	    sum += arr[i];
		
	    // no of positive values
	    if(arr[i] >=0 )
	    	pos++;
		
	    // no of negative values  
	    else
	    	neg++;		
	}
	    
	// if all elements are positive   
	if(pos == n)	
		printf("%lld\n",sum);
	 
	// if all elements are negative   
	else if(neg == n)	
	{
	    // find the largest element
	    for(i = 0; i<n; i++)
	    maxi = max(maxi , arr[i]);
		
            // print the largest elements   
	    printf("%lld\n",maxi);
	}

	else
	{
	    ll int left = 0;
	    ll int maxs = 0;
	        
	    for(i=0; i<n; i++)
	    {	    	
	    	// if variable becomes negative, reset left to 0
	        if( left + arr[i] <=0 )				      
	            left = 0;
	       	            
	        else
		// add if the variable will remain positive
	        {
	            left += arr[i];
	            maxs = max(maxs,left);
		    // keep track of variable's value to know the maximum
	            
	        }	            
	    }

	    // print the answer
	    printf("%lld\n",maxs);
		
	}
}

/*
	Testcase

	Input :

	5
	4 1 -3 7 12

	Output : 21

	Input :

	6
	-1 -4 -5 8 7 9

	Output : 24

	Time complexity : O(n) in average case
	Space complexity : O(1)

*/