Find Immediate Smaller Than X

// { Driver Code Starts
#include <iostream>
using namespace std;

// Position this line where user code will be pasted.

int equilibriumPoint(long long a[], int n);

int main() {

    long long t;
    
    //taking testcases
    cin >> t;

    while (t--) {
        long long n;
        
        //taking input n
        cin >> n;
        long long a[n];

        //adding elements to the array
        for (long long i = 0; i < n; i++) {
            cin >> a[i];
        }

        //calling equilibriumPoint() function
        cout << equilibriumPoint(a, n) << endl;
    }
    return 0;
}
// } Driver Code Ends


// Function to find equilibrium point
// a: input array
// n: size of array
int equilibriumPoint(long long a[], int n) {

    // Your code here
    if(n==1){return 1;}
    else if(n==2){return -1;}
    else{
        long long int presum=0,sum=0;
        for(int i=0;i<n;i++)sum+=a[i];
        for(int i=1;i<n;i++){
            presum+=a[i-1];
            if((sum-a[i])%2==0){
                if(presum==(sum-a[i])/2){
                   // cout<<a[i]<<endl;
                    return (i+1);}
            }
        }
        return -1;
    }
}