#2784 - Check if Array is Good

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Problem Description

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 occurrences of n). For example, base[1] = [1, 1] and base[3] = [1, 2, 3, 3].

Return true if the given array is good, otherwise return false.

Note: A permutation of integers represents an arrangement of these numbers.

Solution

/**
 * @param {number[]} nums
 * @return {boolean}
 */
var isGood = function(nums) {
  const n = Math.max(...nums);
  if (nums.length !== n + 1) return false;

  const frequency = new Array(n + 1).fill(0);
  for (const num of nums) {
    if (num > n) return false;
    frequency[num]++;
  }

  for (let i = 1; i < n; i++) {
    if (frequency[i] !== 1) return false;
  }
  return frequency[n] === 2;
};