Find the largest pair sum in an unsorted array

Given an unsorted of distinct integers, find the largest pair sum in it. For example, the largest pair sum in {12, 34, 10, 6, 40} is 74.

Difficulty Level: Rookie
Expected Time Complexity: O(n) [Only one traversal of array is allowed]

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This problem mainly boils down to finding the largest and second largest element in array. We can find the largest and second largest in O(n) time by traversing array once.

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)

Below is C++ implementation of above algorithm.

// C++ program to find largest pair sum in a given array #include<iostream> 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<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 && 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; }

Output:

Max Pair Sum is 74

Time complexity of above solution is O(n).