You are given an integer array nums. We consider an array good if it is a permutation of an array base[n]. base[n] = [1, 2, ..., n - 1, n, n] (in other words, it is an array of length n + 1 which contains 1 to n - 1 exactly once, plus two occurrence

isGood.js

/**
 * @param {number[]} nums
 * @return {boolean}
 */
var isGood = function(nums) {
    const n = Math.max(...nums);

    // Condition 1: length must be n + 1
    if (nums.length !== n + 1) return false;

    // Count frequencies
    const freq = new Map();
    for (let x of nums) {
        freq.set(x, (freq.get(x) || 0) + 1);
    }

    // Check 1..n-1 appear exactly once
    for (let i = 1; i < n; i++) {
        if (freq.get(i) !== 1) return false;
    }

    // Check n appears exactly twice
    return freq.get(n) === 2;
};