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.
Example 1:
Input: nums = [2, 1, 3] Output: false Explanation: Since the maximum element of the array is 3, the only candidate n for which this array could be a permutation of base[n], is n = 3. However, base[3] has four elements but array nums has three. Therefore, it can not be a permutation of base[3] = [1, 2, 3, 3]. So the answer is false.
Example 2:
Input: nums = [1, 3, 3, 2] Output: true Explanation: Since the maximum element of the array is 3, the only candidate n for which this array could be a permutation of base[n], is n = 3. It can be seen that nums is a permutation of base[3] = [1, 2, 3, 3] (by swapping the second and fourth elements in nums, we reach base[3]). Therefore, the answer is true.
Example 3:
Input: nums = [1, 1] Output: true Explanation: Since the maximum element of the array is 1, the only candidate n for which this array could be a permutation of base[n], is n = 1. It can be seen that nums is a permutation of base[1] = [1, 1]. Therefore, the answer is true.
Example 4:
Input: nums = [3, 4, 4, 1, 2, 1] Output: false Explanation: Since the maximum element of the array is 4, the only candidate n for which this array could be a permutation of base[n], is n = 4. However, base[4] has five elements but array nums has six. Therefore, it can not be a permutation of base[4] = [1, 2, 3, 4, 4]. So the answer is false.
Constraints:
1 <= nums.length <= 1001 <= num[i] <= 200
Solutions
This function checks whether an array is "good" by verifying it contains exactly one occurrence of each number from 1 to n-1, plus exactly two occurrences of n (where n is one less than the array length). It uses a frequency array to count how many times each number appears: it immediately returns false if it encounters any number greater than n, if any number below n appears more than once, or if n itself appears more than twice; otherwise, if all numbers pass these checks, it returns true. In essence, this validates that the input matches the pattern [1, 2, 3, ..., n-1, n, n] in any order.
/**
* @param {number[]} nums
* @return {boolean}
*/
var isGood = function(nums) {
const n = nums.length - 1;
const freq = Array(n + 1).fill(0);
for (const num of nums) {
if (num < n && freq[num] !== 0) return false;
if (num === n && freq[num] === 2) return false;
if (num > n) return false;
freq[num]++;
}
return true;
};