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Check if Array is Good

EasyUpdated: Jul 3, 2025
Practice on:
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Problem

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.

Examples

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 <= 100
  • 1 <= num[i] <= 200

Solution

Method 1 – Frequency Counting

Intuition

The array is good if it is a permutation of [1, 2, ..., n-1, n, n], i.e., it contains all numbers from 1 to n-1 exactly once, and n exactly twice. We can check this by counting the frequency of each number and comparing with the expected pattern.

Approach

  1. Find the maximum value n in the array.
  2. Check if the array length is n + 1 (since base[n] has n+1 elements).
  3. Count the frequency of each number in the array.
  4. For numbers from 1 to n-1, each should appear exactly once.
  5. The number n should appear exactly twice.
  6. If all conditions are satisfied, return true; otherwise, return false.

Code

C++
class Solution {
public:
    bool isGood(vector<int>& nums) {
        int n = *max_element(nums.begin(), nums.end());
        if (nums.size() != n + 1) return false;
        vector<int> cnt(n + 1);
        for (int x : nums) cnt[x]++;
        for (int i = 1; i < n; ++i) if (cnt[i] != 1) return false;
        return cnt[n] == 2;
    }
};
Go
func isGood(nums []int) bool {
    n := 0
    for _, v := range nums {
        if v > n { n = v }
    }
    if len(nums) != n+1 { return false }
    cnt := make([]int, n+1)
    for _, v := range nums { cnt[v]++ }
    for i := 1; i < n; i++ { if cnt[i] != 1 { return false } }
    return cnt[n] == 2
}
Java
class Solution {
    public boolean isGood(int[] nums) {
        int n = 0;
        for (int x : nums) n = Math.max(n, x);
        if (nums.length != n + 1) return false;
        int[] cnt = new int[n + 1];
        for (int x : nums) cnt[x]++;
        for (int i = 1; i < n; ++i) if (cnt[i] != 1) return false;
        return cnt[n] == 2;
    }
}
Kotlin
class Solution {
    fun isGood(nums: IntArray): Boolean {
        val n = nums.maxOrNull() ?: 0
        if (nums.size != n + 1) return false
        val cnt = IntArray(n + 1)
        for (x in nums) cnt[x]++
        for (i in 1 until n) if (cnt[i] != 1) return false
        return cnt[n] == 2
    }
}
Python
class Solution:
    def isGood(self, nums: list[int]) -> bool:
        n = max(nums)
        if len(nums) != n + 1:
            return False
        cnt = [0] * (n + 1)
        for x in nums:
            cnt[x] += 1
        for i in range(1, n):
            if cnt[i] != 1:
                return False
        return cnt[n] == 2
Rust
impl Solution {
    pub fn is_good(nums: Vec<i32>) -> bool {
        let n = *nums.iter().max().unwrap();
        if nums.len() as i32 != n + 1 { return false; }
        let mut cnt = vec![0; (n + 1) as usize];
        for &x in &nums { cnt[x as usize] += 1; }
        for i in 1..n as usize { if cnt[i] != 1 { return false; } }
        cnt[n as usize] == 2
    }
}

Complexity

  • ⏰ Time complexity: O(n), where n is the length of the array.
  • 🧺 Space complexity: O(n) for the frequency map.

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