# 數組中最少的元素 > 原文： [https://www.geeksforgeeks.org/least-frequent-element-array/](https://www.geeksforgeeks.org/least-frequent-element-array/) 給定一個數組，在其中找到最不頻繁的元素。 如果有多個元素出現的次數最少，請打印其中的任何一個。 **示例**： ``` Input : arr[] = {1, 3, 2, 1, 2, 2, 3, 1} Output : 3 3 appears minimum number of times in given array. Input : arr[] = {10, 20, 30} Output : 10 or 20 or 30 ``` 一個簡單的**解決方案**是運行兩個循環。 外循環一一挑選所有元素。 內循環找到所拾取元素的頻率，并與到目前為止的最小值進行比較。 該解決方案的時間復雜度為 `O(n^2)` 更好的**解決方案**是進行排序。 我們首先對數組進行排序，然后線性遍歷數組。 ## C++ ```cpp // CPP program to find the least frequent element // in an array. #include <bits/stdc++.h> using namespace std; int leastFrequent(int arr[], int n) { ????// Sort the array ????sort(arr, arr + n); ????// find the min frequency using linear traversal ????int min_count = n+1, res = -1, curr_count = 1; ????for (int i = 1; i < n; i++) { ????????if (arr[i] == arr[i - 1]) ????????????curr_count++; ????????else { ????????????if (curr_count < min_count) { ????????????????min_count = curr_count; ????????????????res = arr[i - 1]; ????????????} ????????????curr_count = 1; ????????} ????} ????// If last element is least frequent ????if (curr_count < min_count) ????{ ????????min_count = curr_count; ????????res = arr[n - 1]; ????} ????return res; } // driver program int main() { ????int arr[] = {1, 3, 2, 1, 2, 2, 3, 1}; ????int n = sizeof(arr) / sizeof(arr[0]); ????cout << leastFrequent(arr, n); ????return 0; } ``` ## Java ```java // Java program to find the least frequent element // in an array. import java.io.*; import java.util.*; class GFG { ????static int leastFrequent(int arr[], int n) ????{ ????????// Sort the array ????????Arrays.sort(arr); ????????// find the min frequency using? ????????// linear traversal ????????int min_count = n+1, res = -1; ????????int curr_count = 1; ????????for (int i = 1; i < n; i++) { ????????????if (arr[i] == arr[i - 1]) ????????????????curr_count++; ????????????else { ????????????????if (curr_count < min_count) { ????????????????????min_count = curr_count; ????????????????????res = arr[i - 1]; ????????????????} ????????????????curr_count = 1; ????????????} ????????} ????????// If last element is least frequent ????????if (curr_count < min_count) ????????{ ????????????min_count = curr_count; ????????????res = arr[n - 1]; ????????} ????????return res; ????} ????// driver program ????public static void main(String args[]) ????{ ????????int arr[] = {1, 3, 2, 1, 2, 2, 3, 1}; ????????int n = arr.length; ????????System.out.print(leastFrequent(arr, n)); ????} } /*This code is contributed by Nikita Tiwari.*/ ``` ## Python3 ```py # Python 3 program to find the least # frequent element in an array. def leastFrequent(arr, n) : ????# Sort the array ????arr.sort() ????# find the min frequency using ????# linear traversal ????min_count = n + 1 ????res = -1 ????curr_count = 1 ????for i in range(1, n) : ????????if (arr[i] == arr[i - 1]) : ????????????curr_count = curr_count + 1 ????????else : ????????????if (curr_count < min_count) : ????????????????min_count = curr_count ????????????????res = arr[i - 1] ????????????curr_count = 1 ????# If last element is least frequent ????if (curr_count < min_count) : ????????min_count = curr_count ????????res = arr[n - 1] ????return res # Driver program arr = [1, 3, 2, 1, 2, 2, 3, 1] n = len(arr) print(leastFrequent(arr, n)) # This code is contributed # by Nikita Tiwari. ``` ## C# ```cs // C# program to find the least? // frequent element in an array. using System; class GFG { ????static int leastFrequent(int[] arr, int n) ????{ ????????// Sort the array ????????Array.Sort(arr); ????????// find the min frequency? ????????// using linear traversal ????????int min_count = n + 1, res = -1; ????????int curr_count = 1; ????????for (int i = 1; i < n; i++)? ????????{ ????????????if (arr[i] == arr[i - 1]) ????????????????curr_count++; ????????????else? ????????????{ ????????????????if (curr_count < min_count) ????????????????{ ????????????????????min_count = curr_count; ????????????????????res = arr[i - 1]; ????????????????} ????????????????curr_count = 1; ????????????} ????????} ????????// If last element is least frequent ????????if (curr_count < min_count) ????????{ ????????????min_count = curr_count; ????????????res = arr[n - 1]; ????????} ????????return res; ????} ????// Driver code ????static public void Main () ????{ ????????int[] arr = {1, 3, 2, 1, 2, 2, 3, 1}; ????????int n = arr.Length;? ????????// Function calling ????????Console.Write(leastFrequent(arr, n)); ????} } // This code is contributed by Shrikant13 ``` ## PHP ```php <?php // PHP program to find the? // least frequent element // in an array. function leastFrequent($arr, $n) { ????// Sort the array ????sort($arr);? ????sort($arr , $n); ????// find the min frequency? ????// using linear traversal ????$min_count = $n + 1;? ????$res = -1; ????$curr_count = 1; ????for($i = 1; $i < $n; $i++) ????{ ????????if ($arr[$i] == $arr[$i - 1]) ????????????$curr_count++; ????????else? ????????{ ????????????if ($curr_count < $min_count) ????????????{ ????????????????$min_count = $curr_count; ????????????????$res = $arr[$i - 1]; ????????????} ????????????$curr_count = 1; ????????} ????} ????// If last element is? ????// least frequent ????if ($curr_count < $min_count) ????{ ????????$min_count = $curr_count; ????????$res = $arr[$n - 1]; ????} ????return $res; } // Driver Code { ????$arr = array(1, 3, 2, 1, 2, 2, 3, 1); ????$n = sizeof($arr) / sizeof($arr[0]); ????echo leastFrequent($arr, $n); ????return 0; } // This code is contributed by nitin mittal? ?> ``` **輸出**： ``` 3 ``` **時間復雜度**：`O(N log N)` **輔助空間**：`O(1)` **有效解決方案**是使用哈希。 我們創建一個哈希表，并將元素及其頻率計數存儲為鍵值對。 最后，我們遍歷哈希表并打印具有最小值的鍵。 ## C++ ``` // CPP program to find the least frequent element // in an array. #include <bits/stdc++.h> using namespace std; int leastFrequent(int arr[], int n) { ????// Insert all elements in hash. ????unordered_map<int, int> hash; ????for (int i = 0; i < n; i++) ????????hash[arr[i]]++; ????// find the min frequency ????int min_count = n+1, res = -1; ????for (auto i : hash) { ????????if (min_count >= i.second) { ????????????res = i.first; ????????????min_count = i.second; ????????} ????} ????return res; } // driver program int main() { ????int arr[] = {1, 3, 2, 1, 2, 2, 3, 1}; ????int n = sizeof(arr) / sizeof(arr[0]); ????cout << leastFrequent(arr, n); ????return 0; } ``` ## Java ```java //Java program to find the least frequent element //in an array import java.util.HashMap; import java.util.Map; import java.util.Map.Entry; class GFG { ????static int leastFrequent(int arr[],int n) ????{ ????????// Insert all elements in hash. ????????Map<Integer,Integer> count =? ???????????????????new HashMap<Integer,Integer>(); ????????for(int i = 0; i < n; i++) ????????{ ????????????int key = arr[i]; ????????????if(count.containsKey(key)) ????????????{ ????????????????int freq = count.get(key); ????????????????freq++; ????????????????count.put(key,freq); ????????????} ????????????else ????????????????count.put(key,1); ????????} ????????// find min frequency. ????????int min_count = n+1, res = -1; ????????for(Entry<Integer,Integer> val : count.entrySet()) ????????{ ????????????if (min_count >= val.getValue()) ????????????{ ????????????????res = val.getKey(); ????????????????min_count = val.getValue(); ????????????} ????????} ????????return res; ????} ????// driver program ????public static void main (String[] args) { ????????int arr[] = {1, 3, 2, 1, 2, 2, 3, 1}; ????????int n = arr.length; ????????System.out.println(leastFrequent(arr,n)); ????} } // This code is contributed by Akash Singh. ``` ## Python3 ```py # Python3 program to find the most? # frequent element in an array. import math as mt def leastFrequent(arr, n): ????# Insert all elements in Hash. ????Hash = dict() ????for i in range(n): ????????if arr[i] in Hash.keys(): ????????????Hash[arr[i]] += 1 ????????else: ????????????Hash[arr[i]] = 1 ????# find the max frequency ????min_count = n + 1 ????res = -1 ????for i in Hash:? ????????if (min_count >= Hash[i]):? ????????????res = i ????????????min_count = Hash[i] ????return res # Driver Code arr = [1, 3, 2, 1, 2, 2, 3, 1]? n = len(arr) print(leastFrequent(arr, n)) # This code is contributed by # mohit kumar 29 ``` ## C# ``` // C# program to find the? // least frequent element? // in an array. using System; using System.Collections.Generic; class GFG { ????static int leastFrequent(int []arr,? ?????????????????????????????int n) ????{ ????????// Insert all elements in hash. ????????Dictionary<int, int> count =? ????????????????????????new Dictionary<int,? ???????????????????????????????????????int>(); ????????for (int i = 0; i < n; i++) ????????{ ????????????int key = arr[i]; ????????????if(count.ContainsKey(key)) ????????????{ ????????????????int freq = count[key]; ????????????????freq++; ????????????????count[key] = freq; ????????????} ????????????else ????????????????count.Add(key, 1); ????????} ????????// find the min frequency ????????int min_count = n + 1, res = -1; ????????foreach (KeyValuePair<int,? ????????????????????int> pair in count) ????????{ ????????????if (min_count >= pair.Value) ????????????{ ????????????????res = pair.Key; ????????????????min_count = pair.Value; ????????????} ????????}? ????????return res; ????} ????// Driver Code ????static void Main() ????{ ????????int []arr = new int[]{1, 3, 2, 1, ??????????????????????????????2, 2, 3, 1}; ????????int n = arr.Length; ????????Console.Write(leastFrequent(arr, n)); ????} } // This code is contributed by? // Manish Shaw(manishshaw1) ``` **輸出**： ``` 3 ``` **時間復雜度**：`O(n)` **輔助空間**：`O(n)` * * * * * *