Maximum sum subarray such that start and end values are same

https://www.geeksforgeeks.org/maximum-sum-subarray-such-that-start-and-end-values-are-same/

Given an array of N positive numbers, the task is to find a contiguous subarray (L-R) such that a[L]=a[R] and sum of a[L] + a[L+1] +…+ a[R] is maximum.

Examples:

Input: arr[] = {1, 3, 2, 2, 3}
Output: 10
Subarray [3, 2, 2, 3] starts and ends with 3 and has sum = 10

Approach: For every element in the array, let’s find 2 values: First (Leftmost) occurrence in the array and Last (Rightmost) occurrence in the array. Since all numbers are positive, increasing the number of terms can only increase the sum. Hence for every number in the array, we find the sum between it’s leftmost and rightmost occurrence, which can be done quickly using prefix sums. We can keep track of the maximum value found so far and print it in the end.

int maxValue(int a[], int n)

{

    unordered_map<int, int> first, last;

    int pr[n];

    pr[0] = a[0];

  

    for (int i = 1; i <n; i++) {

  

        // Build prefix sum array

        pr[i] = pr[i - 1] + a[i];

  

        // If the value hasn't been encountered before,

        // It is the first occurrence

        if (first[a[i]] == 0)

            first[a[i]] = i;

  

        // Keep updating the last occurrence

        last[a[i]] = i;

    }

  

    int ans = 0;

  

    // Find the maximum sum with same first and last value

    for (int i = 0; i < n; i++) {

        int start = first[a[i]];

        int end = last[a[i]];

        ans = max(ans, pr[end] - pr[start - 1]);

    }

    return ans;

}