在未排序的数组中找到最大的对和

给定一个未排序的不同整数，在其中找到最大的对和。例如，{12，34，10，6，40}中最大的对和为74。
难度等级：新秀
预期时间复杂度：O(n)[仅允许数组遍历一次]

这个问题主要归结为找到数组中最大和第二大元素。通过遍历数组一次，我们可以找到O(n)时间中最大和第二大的值。

1) Initialize both first and second largest
   first = max(arr[0], arr[1])
   second = min(arr[0], arr[1])
2) Loop through remaining elements (from 3rd to end)
   a) If the current element is greater than first, then update first 
       and second. 
   b) Else if the current element is greater than second then update 
    second
3) Return (first + second)

下面是上述算法的实现：

C++

// C++ program to find largest pair sum in a given array
#include
using namespace std;
 
/* Function to return largest pair sum. Assumes that
   there are at-least  two elements in arr[] */
int findLargestSumPair(int arr[], int n)
{
    // Initialize first and second largest element
    int first, second;
    if (arr[0] > arr[1])
    {
        first = arr[0];
        second = arr[1];
    }
    else
    {
        first = arr[1];
        second = arr[0];
    }
 
    // Traverse remaining array and find first and second largest
    // elements in overall array
    for (int i = 2; i first)
        {
            second = first;
            first = arr[i];
        }
 
        /* If arr[i] is in between first and second then update second  */
        else if (arr[i] > second && arr[i] != first)
            second = arr[i];
    }
    return (first + second);
}
 
 
/* Driver program to test above function */
int main()
{
    int arr[] = {12, 34, 10, 6, 40};
    int n = sizeof(arr)/sizeof(arr[0]);
    cout << "Max Pair Sum is " << findLargestSumPair(arr, n);
    return 0;
}

Java

// Java program to find largest pair sum in a given array
 
class Test
{
    static int arr[] = new int[]{12, 34, 10, 6, 40};
     
    /* Method to return largest pair sum. Assumes that
       there are at-least  two elements in arr[] */
    static int findLargestSumPair()
    {
        // Initialize first and second largest element
        int first, second;
        if (arr[0] > arr[1])
        {
            first = arr[0];
            second = arr[1];
        }
        else
        {
            first = arr[1];
            second = arr[0];
        }
      
        // Traverse remaining array and find first and second largest
        // elements in overall array
        for (int i = 2; i first)
            {
                second = first;
                first = arr[i];
            }
      
            /* If arr[i] is in between first and second then update second  */
            else if (arr[i] > second && arr[i] != first)
                second = arr[i];
        }
        return (first + second);
    }
    // Driver method to test the above function
    public static void main(String[] args)
    {
         
        System.out.println("Max Pair Sum is " + findLargestSumPair());
         
    }
}

Python3

# Python3 program to find largest
# pair sum in a given array
 
# Function to return largest pair
# sum. Assumes that there are
# at-least two elements in arr[]
def findLargestSumPair(arr, n):
 
    # Initialize first and second
    # largest element
    if arr[0] > arr[1]:
        first = arr[0]
        second = arr[1]
     
    else:
        first = arr[1]
        second = arr[0]
     
 
    # Traverse remaining array and
    # find first and second largest
    # elements in overall array
    for i in range(2, n):
     
        # If current element is greater
        # than first then update both
        # first and second
        if arr[i] > first:
            second = first
            first = arr[i]
         
 
        # If arr[i] is in between first
        # and second then update second
        elif arr[i] > second and arr[i] != first:
            second = arr[i]
     
    return (first + second)
 
# Driver program to test above function */
arr = [12, 34, 10, 6, 40]
n = len(arr)
print("Max Pair Sum is",
      findLargestSumPair(arr, n))
 
# This code is contributed by Smitha Dinesh Semwal

C#

// C# program to find largest
// pair sum in a given array
using System;
 
class GFG
{
    /* Method to return largest pair
    sum. Assumes that there are
    at-least two elements in arr[] */
    static int findLargestSumPair(int []arr)
    {
        // Initialize first and
        // second largest element
        int first, second;
        if (arr[0] > arr[1])
        {
            first = arr[0];
            second = arr[1];
        }
        else
        {
            first = arr[1];
            second = arr[0];
        }
     
        // Traverse remaining array and
        // find first and second largest
        // elements in overall array
        for (int i = 2; i < arr.Length; i ++)
        {
            /* If current element is greater
               than first then update both
               first and second */
            if (arr[i] > first)
            {
                second = first;
                first = arr[i];
            }
     
            /* If arr[i] is in between first
               and second then update second */
            else if (arr[i] > second &&
                     arr[i] != first)
                second = arr[i];
        }
        return (first + second);
    }
    // Driver Code
    public static void Main()
    {
        int []arr1 = new int[]{12, 34, 10, 6, 40};
        Console.Write("Max Pair Sum is " +
                       findLargestSumPair(arr1));
         
    }
}

PHP

 $arr[1])
    {
        $first = $arr[0];
        $second = $arr[1];
    }
    else
    {
        $first = $arr[1];
        $second = $arr[0];
    }
 
    // Traverse remaining array
    // and find first and second
    // largest elements in overall
    // array
    for ( $i = 2; $i<$n; $i ++)
    {
         
        // If current element is greater
        // than first then update both
        // first and second
        if ($arr[$i] > $first)
        {
            $second = $first;
            $first = $arr[$i];
        }
 
        // If arr[i] is in between first
        // and second then update second
        else if ($arr[$i] > $second and
                 $arr[$i] != $first)
            $second = $arr[$i];
    }
    return ($first + $second);
}
 
    // Driver Code
    $arr = array(12, 34, 10, 6, 40);
    $n = count($arr);
    echo "Max Pair Sum is "
          , findLargestSumPair($arr, $n);
 
// This code is contributed by anuj_67.
?>

Javascript

 
    // Javascript program to find largest
    // pair sum in a given array
     
    /* Method to return largest pair
    sum. Assumes that there are
    at-least two elements in arr[] */
    function findLargestSumPair(arr)
    {
        // Initialize first and
        // second largest element
        let first, second;
        if (arr[0] > arr[1])
        {
            first = arr[0];
            second = arr[1];
        }
        else
        {
            first = arr[1];
            second = arr[0];
        }
       
        // Traverse remaining array and
        // find first and second largest
        // elements in overall array
        for (let i = 2; i < arr.length; i ++)
        {
            /* If current element is greater
               than first then update both
               first and second */
            if (arr[i] > first)
            {
                second = first;
                first = arr[i];
            }
       
            /* If arr[i] is in between first
               and second then update second */
            else if (arr[i] > second &&
                     arr[i] != first)
                second = arr[i];
        }
        return (first + second);
    }
       
    let arr1 = [12, 34, 10, 6, 40];
    document.write("Max Pair Sum is " + findLargestSumPair(arr1));
     
    // This code is contributed by divyeshrabadiy07.

输出 ：

Max Pair Sum is 74

上述解决方案的时间复杂度为O(n)。