Kadane's Algorithm Maximum Subarray in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What problem does Kadane's Algorithm solve?

Kadane's Algorithm finds the maximum sum of a contiguous subarray within a one-dimensional array of numbers.

beginner

What is the main idea behind Kadane's Algorithm?

It keeps track of the current subarray sum and resets it to zero if it becomes negative, while updating the maximum sum found so far.

intermediate

In Kadane's Algorithm, what does the variable 'current_sum' represent?

'current_sum' represents the sum of the current subarray being considered. It resets when the sum becomes negative to start fresh from the next element.

intermediate

Why does Kadane's Algorithm reset the current sum to zero when it becomes negative?

Because a negative sum will only decrease the sum of any future subarray, so it's better to start a new subarray from the next element.

beginner

What is the time complexity of Kadane's Algorithm?

Kadane's Algorithm runs in O(n) time, where n is the number of elements in the array, because it processes each element once.

What does Kadane's Algorithm return?

A Maximum sum of a contiguous subarray

B Minimum sum of a contiguous subarray

C Sum of all elements in the array

D Length of the longest subarray

Kadane's Algorithm finds the maximum sum of any contiguous subarray.

When does Kadane's Algorithm reset the current sum to zero?

A When current sum is positive

B When current sum is zero

C When current sum is negative

D Never resets

It resets current sum to zero when it becomes negative to avoid reducing future sums.

What is the initial value of the maximum sum in Kadane's Algorithm?

A Sum of all elements

B First element of the array

C 0

D Negative infinity or very small number

Maximum sum starts very low to ensure any subarray sum will update it.

What is the time complexity of Kadane's Algorithm?

A O(n)

B O(n log n)

C O(n^2)

D O(log n)

Kadane's Algorithm processes each element once, so it runs in linear time O(n).

Which of these is NOT true about Kadane's Algorithm?

A It uses a running sum and maximum sum variables

B It works only for arrays with positive numbers

C It finds the maximum sum of contiguous subarray

D It can handle negative numbers

Kadane's Algorithm works for arrays with negative numbers as well.

Explain how Kadane's Algorithm finds the maximum sum of a contiguous subarray.

Think about how you decide to keep or drop a subarray while moving through the array.

Describe the role of the variables used in Kadane's Algorithm and how they change during execution.

Focus on how current sum and maximum sum interact as you scan the array.

Practice

1. What does Kadane's Algorithm primarily help to find in an array?

easy

A. The total sum of all elements

B. The minimum element in the array

C. The maximum sum of a contiguous subarray

D. The maximum product of any two elements

Solution

1. Step 1: Understand the problem Kadane's Algorithm solves

   Kadane's Algorithm is designed to find the maximum sum of a contiguous subarray within a one-dimensional array of numbers.

2. Step 2: Compare with other options

   Options B, C, and D describe different problems unrelated to Kadane's Algorithm.

3. Final Answer:

   The maximum sum of a contiguous subarray -> Option C

4. Quick Check:

   Maximum subarray sum = maximum sum of contiguous elements [OK]

Hint: Kadane finds max sum in continuous parts of array [OK]

Common Mistakes:

• Confusing max sum with max product
• Thinking it finds minimum element
• Assuming it sums all elements

2. Which of the following is the correct initialization step in Kadane's Algorithm in C?

easy

A. int max_so_far = INT_MIN, max_ending_here = 0;

B. int max_so_far = 0, max_ending_here = 0;

C. int max_so_far = 0, max_ending_here = INT_MIN;

D. int max_so_far = NULL, max_ending_here = 0;

Solution

1. Step 1: Understand initialization in Kadane's Algorithm

   We initialize max_so_far to the smallest integer (INT_MIN) to handle arrays with all negative numbers correctly, and max_ending_here to 0 to start accumulating sums.

2. Step 2: Evaluate each option

   int max_so_far = 0, max_ending_here = 0; initializes max_so_far to 0, which fails for all negative arrays. int max_so_far = 0, max_ending_here = INT_MIN; incorrectly initializes max_ending_here to INT_MIN. int max_so_far = NULL, max_ending_here = 0; uses NULL which is invalid for int.

3. Final Answer:

   int max_so_far = INT_MIN, max_ending_here = 0; -> Option A

4. Quick Check:

   Use INT_MIN for max_so_far to handle negatives [OK]

Hint: Start max_so_far at INT_MIN to handle negatives [OK]

Common Mistakes:

• Initializing max_so_far to 0 causes wrong results
• Using NULL for integer variables
• Swapping initial values of variables

3. What is the output of the following C code using Kadane's Algorithm?

int arr[] = {-2, 3, 2, -1};
int n = 4;
int max_so_far = INT_MIN, max_ending_here = 0;
for (int i = 0; i < n; i++) {
    max_ending_here += arr[i];
    if (max_so_far < max_ending_here) max_so_far = max_ending_here;
    if (max_ending_here < 0) max_ending_here = 0;
}
printf("%d", max_so_far);

medium

A. 4

B. 5

C. 3

D. -1

Solution

1. Step 1: Trace the loop and update sums

   Iteration 0: max_ending_here = 0 + (-2) = -2; max_so_far = max(INT_MIN, -2) = -2; max_ending_here < 0, reset to 0.
   Iteration 1: max_ending_here = 0 + 3 = 3; max_so_far = max(-2, 3) = 3; max_ending_here >= 0, no reset.
   Iteration 2: max_ending_here = 3 + 2 = 5; max_so_far = max(3, 5) = 5; max_ending_here >= 0, no reset.
   Iteration 3: max_ending_here = 5 + (-1) = 4; max_so_far = max(5, 4) = 5; max_ending_here >= 0, no reset.

2. Step 2: Final max_so_far value

   After loop ends, max_so_far is 5, which is the maximum subarray sum.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Max subarray sum = 5 [OK]

Hint: Add elements, reset sum if negative, track max sum [OK]

Common Mistakes:

• Not resetting max_ending_here when negative
• Confusing max_so_far with current sum
• Incorrectly summing non-contiguous elements

4. Identify the bug in this Kadane's Algorithm implementation in C:

int maxSubArraySum(int arr[], int n) {
    int max_so_far = 0, max_ending_here = 0;
    for (int i = 0; i < n; i++) {
        max_ending_here += arr[i];
        if (max_so_far < max_ending_here) max_so_far = max_ending_here;
        if (max_ending_here < 0) max_ending_here = 0;
    }
    return max_so_far;
}

medium

A. max_so_far should be initialized to INT_MIN

B. max_ending_here should be initialized to INT_MIN

C. Loop should start from i = 1

D. Return statement should return max_ending_here

Solution

1. Step 1: Analyze initialization of max_so_far

   Initializing max_so_far to 0 fails when all array elements are negative, as max_so_far will never update to a negative number.

2. Step 2: Correct initialization and effect

   Setting max_so_far to INT_MIN ensures the algorithm correctly finds the maximum subarray sum even if all numbers are negative.

3. Final Answer:

   max_so_far should be initialized to INT_MIN -> Option A

4. Quick Check:

   Use INT_MIN for max_so_far to handle negative arrays [OK]

Hint: Initialize max_so_far to INT_MIN for negative arrays [OK]

Common Mistakes:

• Initializing max_so_far to 0 causes wrong max for negatives
• Changing loop start index unnecessarily
• Returning wrong variable

5. Given an array int arr[] = {-3, -2, -1, -4}, what is the maximum subarray sum using Kadane's Algorithm?

hard

A. -3

B. 0

C. -10

D. -1

Solution

1. Step 1: Understand Kadane's behavior with all negative numbers

   When all numbers are negative, resetting max_ending_here to 0 loses the maximum negative number. Proper Kadane's initialization with max_so_far = INT_MIN helps track the largest negative number.

2. Step 2: Trace the array values

   max_so_far starts at INT_MIN.
   i=0: max_ending_here = -3; max_so_far = -3; reset max_ending_here to 0 since negative.
   i=1: max_ending_here = -2; max_so_far = max(-3, -2) = -2; reset max_ending_here to 0.
   i=2: max_ending_here = -1; max_so_far = max(-2, -1) = -1; reset max_ending_here to 0.
   i=3: max_ending_here = -4; max_so_far = max(-1, -4) = -1; reset max_ending_here to 0.

3. Final Answer:

   -1 -> Option D

4. Quick Check:

   Max negative number = -1 [OK]

Hint: Max subarray in negatives is max single element [OK]

Common Mistakes:

• Returning 0 instead of max negative
• Not handling all negative arrays
• Assuming sum must be positive