在对未排序的数组进行排序时查找首次出现的索引

给定一个未排序的数组和一个数字x，请在对数组进行排序时找到x首次出现的索引。如果不存在x，则打印-1。
例子：

Input : arr[] = {10, 30, 20, 50, 20}
           x = 20
Output : 1
Sorted array is {10, 20, 20, 30, 50}

Input : arr[] = {10, 30, 20, 50, 20}
           x = 60
Output : -1
60 is not present in array. 

一个简单的解决方案是先对数组排序，然后进行二进制搜索以找到第一个匹配项。

C++

// CPP program to find index of first
// occurrence of x when array is sorted.
#include 
using namespace std;
 
int findFirst(int arr[], int n, int x)
{
    sort(arr, arr + n);
 
    // lower_bound returns iterator pointing to
    // first element that does not compare less
    // to x.
    int* ptr = lower_bound(arr, arr + n, x);
 
    // If x is not present return -1.
    return (*ptr != x) ? -1 : (ptr - arr);
}
 
int main()
{
    int x = 20, arr[] = { 10, 30, 20, 50, 20 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << findFirst(arr, n, x);
    return 0;
}

Java

// Java program to find index of first
// occurrence of x when array is sorted.
import java.util.*;
 
class GFG {
    static int findFirst(int arr[], int n, int x)
    {
        Arrays.sort(arr);
 
        // lower_bound returns iterator pointing to
        // first element that does not compare less
        // to x.
        int ptr = lowerBound(arr, 0, n, x);
 
        // If x is not present return -1.
        return (arr[ptr] != x) ? -1 : (ptr);
    }
 
    static int lowerBound(int[] a, int low, int high,
                          int element)
    {
        while (low < high) {
            int middle = low + (high - low) / 2;
            if (element > a[middle])
                low = middle + 1;
            else
                high = middle;
        }
        return low;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int x = 20, arr[] = { 10, 30, 20, 50, 20 };
        int n = arr.length;
        System.out.println(findFirst(arr, n, x));
    }
}
 
// This code is contributed by 29AjayKumar

Python3

# Python3 program to find index of first
# occurrence of x when array is sorted.
import math
 
 
def findFirst(arr, n, x):
    arr.sort()
 
    # lower_bound returns iterator pointing to
    # first element that does not compare less
    # to x.
    ptr = lowerBound(arr, 0, n, x)
 
    # If x is not present return -1.
    return 1 if (ptr != x) else (ptr - arr)
 
 
def lowerBound(a, low, high, element):
    while(low < high):
        middle = low + (high - low) // 2
        if(element > a[middle]):
            low = middle + 1
        else:
            high = middle
    return low
 
 
# Driver Code
if __name__ == '__main__':
    x = 20
    arr = [10, 30, 20, 50, 20]
    n = len(arr)
    print(findFirst(arr, n, x))
 
# This code is contributed by Rajput-Ji

C#

// C# program to find index of first
// occurrence of x when array is sorted.
using System;
 
class GFG {
    static int findFirst(int[] arr, int n, int x)
    {
        Array.Sort(arr);
 
        // lower_bound returns iterator pointing to
        // first element that does not compare less
        // to x.
        int ptr = lowerBound(arr, 0, n, x);
 
        // If x is not present return -1.
        return (arr[ptr] != x) ? -1 : (ptr);
    }
 
    static int lowerBound(int[] a, int low, int high,
                          int element)
    {
        while (low < high) {
            int middle = low + (high - low) / 2;
            if (element > a[middle])
                low = middle + 1;
            else
                high = middle;
        }
        return low;
    }
 
    // Driver Code
    static public void Main()
    {
        int x = 20;
        int[] arr = { 10, 30, 20, 50, 20 };
        int n = arr.Length;
        Console.Write(findFirst(arr, n, x));
    }
}
 
// This code is contributed by ajit.

输出 ：

1

时间复杂度： O(n Log n)
一个有效的解决方案是简单地计算比x小的元素。

C++

// CPP program to find index of first
// occurrence of x when array is sorted.
#include 
using namespace std;
 
int findFirst(int arr[], int n, int x)
{
    int count = 0;
    bool isX = false;
    for (int i = 0; i < n; i++) {
        if (arr[i] == x)
            isX = true;
        else if (arr[i] < x)
            count++;
    }
    return (isX == false) ? -1 : count;
}
 
int main()
{
    int x = 20, arr[] = { 10, 30, 20, 50, 20 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << findFirst(arr, n, x);
    return 0;
}

Java

// Java program to find index of first
// occurrence of x when array is sorted.
 
public class GFG {
 
    static int findFirst(int arr[], int n, int x)
    {
        int count = 0;
        boolean isX = false;
        for (int i = 0; i < n; i++) {
            if (arr[i] == x) {
                isX = true;
            }
            else if (arr[i] < x) {
                count++;
            }
        }
        return (isX == false) ? -1 : count;
    }
 
    // Driver main
    public static void main(String[] args)
    {
        int x = 20, arr[] = { 10, 30, 20, 50, 20 };
        int n = arr.length;
        System.out.println(findFirst(arr, n, x));
    }
}
/*This code is contributed by PrinciRaj1992*/

的Python 3

# Python 3 program to find index
# of first occurrence of x when
# array is sorted.
 
 
def findFirst(arr, n, x):
 
    count = 0
    isX = False
    for i in range(n):
        if (arr[i] == x):
            isX = True
        elif (arr[i] < x):
            count += 1
 
    return -1 if(isX == False) else count
 
 
# Driver Code
if __name__ == "__main__":
    x = 20
    arr = [10, 30, 20, 50, 20]
    n = len(arr)
    print(findFirst(arr, n, x))
 
# This code is contributed
# by ChitraNayal

C＃

// C# program to find index of first
// occurrence of x when array is sorted.
using System;
 
public class GFG {
 
    static int findFirst(int[] arr, int n, int x)
    {
        int count = 0;
        bool isX = false;
        for (int i = 0; i < n; i++) {
            if (arr[i] == x) {
                isX = true;
            }
            else if (arr[i] < x) {
                count++;
            }
        }
        return (isX == false) ? -1 : count;
    }
 
    // Driver main
    public static void Main()
    {
        int x = 20;
        int[] arr = { 10, 30, 20, 50, 20 };
        int n = arr.Length;
        Console.WriteLine(findFirst(arr, n, x));
    }
}
/*This code is contributed by PrinciRaj1992*/

输出 ：

1

时间复杂度： O(N)