非重复元素

在给定的整数数组中找到第一个非重复元素。

例子：

Input : -1 2 -1 3 2
Output : 3
Explanation : The first number that does not 
repeat is : 3

Input : 9 4 9 6 7 4
Output : 6

一个简单的解决方案是使用两个循环。外循环一个接一个地选择元素，内循环检查该元素是否多次出现。

C++

// Simple CPP program to find first non-
// repeating element.
#include 
using namespace std;
  
int firstNonRepeating(int arr[], int n)
{
    for (int i = 0; i < n; i++) {
        int j;
        for (j = 0; j < n; j++)
            if (i != j && arr[i] == arr[j])
                break;
        if (j == n)
            return arr[i];
    }
    return -1;
}
  
// Driver code
int main()
{
    int arr[] = { 9, 4, 9, 6, 7, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << firstNonRepeating(arr, n);
    return 0;
}

Java

// Java program to find first non-repeating
// element.
class GFG {
  
    static int firstNonRepeating(int arr[], int n)
    {
        for (int i = 0; i < n; i++) {
            int j;
            for (j = 0; j < n; j++)
                if (i != j && arr[i] == arr[j])
                    break;
            if (j == n)
                return arr[i];
        }
  
        return -1;
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        int arr[] = { 9, 4, 9, 6, 7, 4 };
        int n = arr.length;
  
        System.out.print(firstNonRepeating(arr, n));
    }
}
  
// This code is contributed by Anant Agarwal.

Python3

# Python3 program to find first 
# non-repeating element.
  
def firstNonRepeating(arr, n):
  
    for i in range(n):
        j = 0
        while(j < n):
            if (i != j and arr[i] == arr[j]):
                break
            j += 1
        if (j == n):
            return arr[i]
      
    return -1
      
# Driver code
arr = [ 9, 4, 9, 6, 7, 4 ]
n = len(arr)
print(firstNonRepeating(arr, n))
  
# This code is contributed by Anant Agarwal.

C#

// C# program to find first non-
// repeating element.
using System;
  
class GFG {
    static int firstNonRepeating(int[] arr, int n)
    {
        for (int i = 0; i < n; i++) {
            int j;
            for (j = 0; j < n; j++)
                if (i != j && arr[i] == arr[j])
                    break;
            if (j == n)
                return arr[i];
        }
        return -1;
    }
  
    // Driver code
    public static void Main()
    {
        int[] arr = { 9, 4, 9, 6, 7, 4 };
        int n = arr.Length;
        Console.Write(firstNonRepeating(arr, n));
    }
}
// This code is contributed by Anant Agarwal.

输出：

6

一个有效的解决方案是使用哈希。
1)在哈希表中遍历数组和插入元素及其计数。
2)再次遍历数组，并打印计数等于1的第一个元素。

C++

// Efficient CPP program to find first non-
// repeating element.
#include 
using namespace std;
  
int firstNonRepeating(int arr[], int n)
{
    // Insert all array elements in hash
    // table
    unordered_map mp;
    for (int i = 0; i < n; i++)
        mp[arr[i]]++;
  
    // Traverse array again and return
    // first element with count 1.
    for (int i = 0; i < n; i++)
        if (mp[arr[i]] == 1)
            return arr[i];
    return -1;
}
  
// Driver code
int main()
{
    int arr[] = { 9, 4, 9, 6, 7, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << firstNonRepeating(arr, n);
    return 0;
}

Java

// Efficient Java program to find first non-
// repeating element.
import java.util.*;
  
class GFG {
  
    static int firstNonRepeating(int arr[], int n)
    {
        // Insert all array elements in hash
        // table
  
        Map m = new HashMap<>();
        for (int i = 0; i < n; i++) {
            if (m.containsKey(arr[i])) {
                m.put(arr[i], m.get(arr[i]) + 1);
            }
            else {
                m.put(arr[i], 1);
            }
        }
        // Traverse array again and return
        // first element with count 1.
        for (int i = 0; i < n; i++)
            if (m.get(arr[i]) == 1)
                return arr[i];
        return -1;
    }
  
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 9, 4, 9, 6, 7, 4 };
        int n = arr.length;
        System.out.println(firstNonRepeating(arr, n));
    }
}
  
// This code contributed by Rajput-Ji

Python3

# Efficient Python3 program to find first 
# non-repeating element.
from collections import defaultdict
  
def firstNonRepeating(arr, n):
    mp = defaultdict(lambda:0)
  
    # Insert all array elements in hash table 
    for i in range(n):
        mp[arr[i]] += 1
  
    # Traverse array again and return 
    # first element with count 1. 
    for i in range(n):
        if mp[arr[i]] == 1:
            return arr[i]
    return -1
  
# Driver Code
arr = [9, 4, 9, 6, 7, 4]
n = len(arr)
print(firstNonRepeating(arr, n)) 
  
# This code is contributed by Shrikant13

C＃

// Efficient C# program to find first non-
// repeating element.
using System;
using System.Collections.Generic;
  
class GFG {
  
    static int firstNonRepeating(int[] arr, int n)
    {
        // Insert all array elements in hash
        // table
  
        Dictionary m = new Dictionary();
        for (int i = 0; i < n; i++) {
            if (m.ContainsKey(arr[i])) {
                var val = m[arr[i]];
                m.Remove(arr[i]);
                m.Add(arr[i], val + 1);
            }
            else {
                m.Add(arr[i], 1);
            }
        }
  
        // Traverse array again and return
        // first element with count 1.
        for (int i = 0; i < n; i++)
            if (m[arr[i]] == 1)
                return arr[i];
        return -1;
    }
  
    // Driver code
    public static void Main(String[] args)
    {
        int[] arr = { 9, 4, 9, 6, 7, 4 };
        int n = arr.Length;
        Console.WriteLine(firstNonRepeating(arr, n));
    }
}
  
// This code has been contributed by 29AjayKumar

输出：

6

时间复杂度： O(n)
辅助空间： O(n)

进一步的优化：如果数组有很多重复项，我们还可以使用值是一对的哈希表将索引存储在哈希表中。现在，我们只需要遍历哈希表(不是完整的数组)中的键来查找第一个非重复的键。

打印所有非重复元素：

C++

// Efficient CPP program to print all non-
// repeating elements.
#include 
using namespace std;
  
void firstNonRepeating(int arr[], int n)
{
    // Insert all array elements in hash
    // table
    unordered_map mp;
    for (int i = 0; i < n; i++)
        mp[arr[i]]++;
  
    // Traverse through map only and
    for (auto x : mp)
        if (x.second == 1)
            cout << x.first << " ";
}
  
// Driver code
int main()
{
    int arr[] = { 9, 4, 9, 6, 7, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    firstNonRepeating(arr, n);
    return 0;
}

Java

// Efficient Java program to print all non-
// repeating elements.
import java.util.*;
  
class GFG {
  
    static void firstNonRepeating(int arr[], int n)
    {
        // Insert all array elements in hash
        // table
        Map m = new HashMap<>();
        for (int i = 0; i < n; i++) {
            if (m.containsKey(arr[i])) {
                m.put(arr[i], m.get(arr[i]) + 1);
            }
            else {
                m.put(arr[i], 1);
            }
        }
  
        // Traverse through map only and
        // using for-each loop for iteration over Map.entrySet()
        for (Map.Entry x : m.entrySet())
            if (x.getValue() == 1)
                System.out.print(x.getKey() + " ");
    }
  
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 9, 4, 9, 6, 7, 4 };
        int n = arr.length;
        firstNonRepeating(arr, n);
    }
}
  
// This code has been contributed by 29AjayKumar

Python3

# Efficient Python program to print all non-
# repeating elements.
  
def firstNonRepeating(arr, n):
      
    # Insert all array elements in hash
    # table
    mp={}
    for i in range(n):
        if arr[i] not in mp:
            mp[arr[i]]=0
        mp[arr[i]]+=1
          
    # Traverse through map only and
    for x in mp:
        if (mp[x]== 1):
            print(x,end=" ")
              
# Driver code
arr = [ 9, 4, 9, 6, 7, 4 ]
n = len(arr)
firstNonRepeating(arr, n)
   
# This code is contributed by shivanisinghss2110

C＃

// Efficient C# program to print all non-
// repeating elements.
using System;
using System.Collections.Generic;
  
class GFG {
  
    static void firstNonRepeating(int[] arr, int n)
    {
        // Insert all array elements in hash
        // table
        Dictionary m = new Dictionary();
        for (int i = 0; i < n; i++) {
            if (m.ContainsKey(arr[i])) {
                var val = m[arr[i]];
                m.Remove(arr[i]);
                m.Add(arr[i], val + 1);
            }
            else {
                m.Add(arr[i], 1);
            }
        }
  
        // Traverse through map only and
        // using for-each loop for iteration over Map.entrySet()
        foreach(KeyValuePair x in m)
        {
            if (x.Value == 1) {
                Console.Write(x.Key + " ");
            }
        }
    }
  
    // Driver code
    public static void Main(String[] args)
    {
        int[] arr = { 9, 4, 9, 6, 7, 4 };
        int n = arr.Length;
        firstNonRepeating(arr, n);
    }
}
  
/* This code contributed by PrinciRaj1992 */

输出：

7 6