# Find Pivot Index

<https://leetcode.com/contest/weekly-contest-58/problems/find-pivot-index/>

> Given an array of integers`nums`, write a method that returns the "pivot" index of this array.
>
> We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.
>
> If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.

## Thoughts

看到要对subarry sum怎样怎样自然想到presum.

## Code

```
class Solution {
    public int pivotIndex(int[] nums) {
        int[] presum = new int[nums.length];
        int sum = 0;
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            presum[i] = sum;
        }

        for (int i = 0; i < nums.length; i++) {
            int leftSum = i > 0 ? presum[i - 1] : 0;
            int rightSum = i < nums.length - 1 ? sum - presum[i]: 0;
            if (leftSum == rightSum) {
                return i;
            }
        }
        return -1;
    }
}
```

## Analysis

做题耗时6min

时空复杂度都是O(N).
