2221. Find Triangular Sum of an Array

T:O(n^2)

S:O(n^2)

class Solution {
    public int triangularSum(int[] nums) {
        List<Integer> list = new ArrayList<>();
        for (int num : nums) {
            list.add(num);
        }
        List<Integer> temp = new ArrayList<>();
        while (list.size() != 1) {
            for (int i = 0; i < list.size()-1; i++) {
                temp.add((list.get(i) + list.get(i+1))%10);
            }
            list = temp;
            temp = new ArrayList<>();
        }
        return list.get(0);
    }
}

T:O(n^2)

S:O(1)

```java
class Solution {
    public int triangularSum(int[] nums) {
        int n = nums.length;
        while (n > 1) {
            for (int i = 0; i < n-1; i++) {
                nums[i] = (nums[i] + nums[i+1])%10;
            }
            n--;
        }
        return nums[0];
    }
}
// still T: O(n^2) but space: O(1) in place
```