第一个元素在数组中出现 k 次

给定一个由 n 个整数组成的数组。任务是找到出现 k 次的第一个元素。如果没有元素出现k次打印-1。整数元素的分布可以在任何范围内。
例子：

Input: {1, 7, 4, 3, 4, 8, 7},
k = 2
Output: 7
Both 7 and 4 occur 2 times.
But 7 is the first that occurs 2 times.

Input: {4, 1, 6, 1, 6, 4},
k = 1
Output: -1

编程需要懂一点英语

简单的方法：通过使用两个循环，计算一个数字在数组中出现的次数。
时间复杂度：O(n ² )。
有效方法：使用 unordered_map 进行散列，因为范围未知。脚步：

从左到右遍历数组元素。
在遍历时增加它们在哈希表中的计数。
再次从左到右遍历数组并检查哪个元素的计数等于 k。打印该元素并停止。
如果没有元素的计数等于 k，则打印 -1。

以下是上述方法的试运行：

下面是上述方法的实现：

C++

// C++ implementation to find first
// element occurring k times
#include 
 
using namespace std;
 
// function to find the first element
// occurring k number of times
int firstElement(int arr[], int n, int k)
{
    // unordered_map to count
    // occurrences of each element
    unordered_map count_map;
    for (int i=0; i




Java

import java.util.HashMap;
 
// Java implementation to find first
// element occurring k times
class GFG {
 
// function to find the first element
// occurring k number of times
    static int firstElement(int arr[], int n, int k) {
        // unordered_map to count
        // occurrences of each element
 
        HashMap count_map = new HashMap<>();
        for (int i = 0; i < n; i++) {
            int a = 0;
            if(count_map.get(arr[i])!=null){
                a = count_map.get(arr[i]);
            }
             
            count_map.put(arr[i], a+1);
        }
        //count_map[arr[i]]++;
 
        for (int i = 0; i < n; i++) // if count of element == k ,then
        // it is the required first element
        {
            if (count_map.get(arr[i]) == k) {
                return arr[i];
            }
        }
 
        // no element occurs k times
        return -1;
    }
 
// Driver program to test above
    public static void main(String[] args) {
        int arr[] = {1, 7, 4, 3, 4, 8, 7};
        int n = arr.length;
        int k = 2;
        System.out.println(firstElement(arr, n, k));
    }
}
 
//this code contributed by Rajput-Ji



Python3

# Python3 implementation to
# find first element
# occurring k times
 
# function to find the
# first element occurring
# k number of times
def firstElement(arr, n, k):
 
    # dictionary to count
    # occurrences of
    # each element
    count_map = {};
    for i in range(0, n):
        if(arr[i] in count_map.keys()):
            count_map[arr[i]] += 1
        else:
            count_map[arr[i]] = 1
        i += 1
     
    for i in range(0, n):
         
        # if count of element == k ,
        # then it is the required
        # first element
        if (count_map[arr[i]] == k):
            return arr[i]
        i += 1
             
    # no element occurs k times
    return -1
 
# Driver Code
if __name__=="__main__":
 
    arr = [1, 7, 4, 3, 4, 8, 7];
    n = len(arr)
    k = 2
    print(firstElement(arr, n, k))
 
# This code is contributed
# by Abhishek Sharma



C#

// C# implementation to find first
// element occurring k times
using System;
using System.Collections.Generic;
 
class GFG
{
 
    // function to find the first element
    // occurring k number of times
    static int firstElement(int []arr, int n, int k)
    {
        // unordered_map to count
        // occurrences of each element
 
        Dictionary count_map = new Dictionary();
        for (int i = 0; i < n; i++)
        {
            int a = 0;
            if(count_map.ContainsKey(arr[i]))
            {
                a = count_map[arr[i]];
                count_map.Remove(arr[i]);
                count_map.Add(arr[i], a+1);
            }
            else
                count_map.Add(arr[i], 1);
        }
        //count_map[arr[i]]++;
 
        for (int i = 0; i < n; i++) // if count of element == k ,then
        // it is the required first element
        {
            if (count_map[arr[i]] == k)
            {
                return arr[i];
            }
        }
 
        // no element occurs k times
        return -1;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        int []arr = {1, 7, 4, 3, 4, 8, 7};
        int n = arr.Length;
        int k = 2;
        Console.WriteLine(firstElement(arr, n, k));
    }
}
 
// This code has been contributed by 29AjayKumar



Javascript