You are given an integer array nums. A subarray is called balanced if the number of distinct even numbers in the subarray is equal to the number of distinct odd numbers. Return the length of the longest balanced subarray.

longestBalanced.js

/**
 * @param {number[]} nums
 * @return {number}
 */
var longestBalanced = function(nums) {
    const n = nums.length;
    let maxLen = 0;

    // Fix a starting index i
    for (let i = 0; i < n; i++) {
        // For each new start, we reset the distinct sets
        const evens = new Set();
        const odds = new Set();

        // Extend the subarray to the right: j is the ending index
        for (let j = i; j < n; j++) {
            const x = nums[j];

            // Track distinct evens and odds in the current subarray nums[i..j]
            if (x % 2 === 0) {
                evens.add(x);
            } else {
                odds.add(x);
            }

            // Check if this subarray is balanced
            if (evens.size === odds.size) {
                const len = j - i + 1;
                if (len > maxLen) {
                    maxLen = len;
                }
            }
        }
    }

    return maxLen;
};