Example: Prefix Sum Equilibrium Index

// Prefix Sum Equilibrium Index
#include <stdio.h>

int findEquilibriumIndexPrefixSum(int arr[], int n) {
    if (n == 0) return -1; // Handle empty array

    long long totalSum = 0;
    // Step 1: Calculate the total sum of the array
    for (int i = 0; i < n; i++) {
        totalSum += arr[i];
    }

    long long leftSum = 0;
    // Step 2: Iterate through the array to find equilibrium index
    for (int i = 0; i < n; i++) {
        // rightSum is totalSum - leftSum - current element
        long long rightSum = totalSum - leftSum - arr[i];

        if (leftSum == rightSum) {
            return i; // Found an equilibrium index
        }
        // Update leftSum for the next iteration
        leftSum += arr[i];
    }
    return -1; // No equilibrium index found
}

int main() {
    // Step 1: Define an array
    int arr[] = {-7, 1, 5, 2, -4, 3, 0};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Step 2: Find the equilibrium index using prefix sum
    int equilibriumIndex = findEquilibriumIndexPrefixSum(arr, n);

    // Step 3: Print the result
    if (equilibriumIndex != -1) {
        printf("Equilibrium index found at position: %d\n", equilibriumIndex);
    } else {
        printf("No equilibrium index found.\n");
    }

    // Test with another array
    int arr2[] = {1, 2, 3};
    int n2 = sizeof(arr2) / sizeof(arr2[0]);
    equilibriumIndex = findEquilibriumIndexPrefixSum(arr2, n2);
    if (equilibriumIndex != -1) {
        printf("Equilibrium index for {1, 2, 3} found at position: %d\n", equilibriumIndex);
    } else {
        printf("No equilibrium index for {1, 2, 3} found.\n");
    }

    // Test with an array where 0 is equilibrium
    int arr3[] = {0, 0, 0};
    int n3 = sizeof(arr3) / sizeof(arr3[0]);
    equilibriumIndex = findEquilibriumIndexPrefixSum(arr3, n3);
    if (equilibriumIndex != -1) {
        printf("Equilibrium index for {0, 0, 0} found at position: %d\n", equilibriumIndex);
    } else {
        printf("No equilibrium index for {0, 0, 0} found.\n");
    }

    return 0;
}