Good Subarrays Easy

Good Subarrays Easy

Problem Statement:

A subarray is called "good" if its length is even and sum is even. Count all good subarrays in array A.

Examples:

Example 1:

Input: A = 1, 2, 3, 4

Output: 2

Explanation: 1,2,3,4 has even length and sum=10 (even), 2,3,4 has odd length (not good)

Approach:

Check all subarrays with even length, calculate sum, count if even.

Time Complexity:

• Time = O(N²), Space = O(1)

JavaScript Code:

function countGoodSubarrays(A) {
    let count = 0;
    const N = A.length;
    
    for (let start = 0; start < N; start++) {
        let sum = 0;
        for (let end = start; end < N; end++) {
            sum += A[end];
            const length = end - start + 1;
            
            if (length % 2 === 0 && sum % 2 === 0) {
                count++;
            }
        }
    }
    
    return count;
}

Key Takeaways:

1. Multiple conditions require careful checking.
2. Track running sum to avoid recalculation.
3. Check both length and sum properties.
4. O(N²) when checking all subarrays.
5. Optimization possible with math properties.