Solution: if there's only one element in the array, return that index.

There are 3 cases:

1. [1,2] return index 0 (first if condition)

2. [2,1] return index 0 (second if condition)

3. [1,3,2,1] return 1 (last if condition)

Code:

```
public int findPeakElement(int[] nums) {
if(nums.length ==1) return 0;
for(int i=0;i<=nums.length-1;i++){
if(i==0){
if(nums[i]>nums[i+1]) return i; //return 0 for [2,1]
} else if (i == nums.length-1){
if(nums[i]>nums[i-1]) return i; //return 0 for [1,2]
} else if(nums[i] > nums[i-1] && nums[i] > nums[i+1])return i;
}
return 0;
}
```