Balance Index

• Time : O(n)
• Space : O(1)

int balanceIndex(vector<int> array) {
    long long r = accumulate(begin(array), end(array), 0ll), n = array.size(), l = 0;
    for(int i = 0; i < n; i++) {
        r -= array[i];
        if(l == r) return i;
        l += array[i];
    }
    return -1;
}