[FIXED] How to improve performance for this leetcode solution?

Issue

I am solving this Leetcode problem: https://leetcode.com/problems/find-pivot-index/ And I came up with this solution:

import java.util.ArrayList;
import java.util.List;

public class Solution {
    public int pivotIndex(int[] nums) {
        Integer result = null;
        List<Integer> right = new ArrayList<>(nums.length);
        List<Integer> left = new ArrayList<>(nums.length);
        int leftSum, rightSum;

        for (int i = 0; i < nums.length; i++) {
            leftSum = 0;
            rightSum = 0;

            for (int j = 0; j < i; j++) {
                left.add(nums[j]);
            }

            for (int num : left) {
                leftSum += num;
            }

            for (int j = i + 1; j < nums.length; j++) {
                if (j > nums.length) {
                    right.add(0);
                } else {
                    right.add(nums[j]);
                }
            }

            for (int num : right) {
                rightSum += num;
            }

            if (leftSum == rightSum) {
                result = i;
                return result;
            } else {
                result = -1;
                left.clear();
                right.clear();
            }
        }
        return result;
    }
}

But I'm getting Time Limit Exceeded... Can someone help me with some advise on how to make this run faster?
Before, I was instantiating a new ArrayList object at the start of the first for loop, so I changed their scope so that only one instantiation happens and just cleared the ArrayLists at the end of the for loop.
Same for leftSum and rightSum, I changed their scope for the whole method and just change their value to 0 at the start of the first for loop. I figured both these changed would make it faster but apparently didn't?
My code is slow somewhere else I can't detect it right now.

Any tips / good practices would be highly appreciated as someone who's trying to prepare for the first job interviews in this field :)

Solution

For anyone curious, I ended up creating another solution to this.

class Solution {
    public int pivotIndex(int[] nums) {
        int leftSum = 0, sum = Arrays.stream(nums).sum();

        for (int i = 0; i < nums.length; i++) {
            if (i != 0) {
                leftSum += nums[i - 1];
            }
            if (leftSum == sum - leftSum - nums[i]) {
                return i;
            }
        }
        return -1;
    }
}

For me is easier to understand than the other answers.

Answered By - Santiagolp

Answer Checked By - Robin (PHPFixing Admin)