Column Sums in Java

Given a grid of integers grid, return an ArrayList with the sum of each column, from left to right.

A grid is a table of numbers made of rows. Every row has the same number of values, one in each column. Rows are counted from the top and columns from the left, both starting at 0.

Column c is made of the value at position c in every row.

Example 1:

Input: grid = [[1, 2, 3],
               [4, 5, 6]]
Output: [5, 7, 9]
Explanation: Column 0 holds 1 and 4, column 1 holds 2 and 5, and column 2 holds 3 and 6: 1 + 4 = 5, 2 + 5 = 7 and 3 + 6 = 9.

Example 2:

Input: grid = [[2, -1],
               [3, -1],
               [-5, 7]]
Output: [0, 5]
Explanation: Column 0 is 2 + 3 + (-5) = 0 and column 1 is -1 + (-1) + 7 = 5.

Example 3:

Input: grid = [[4, -2, 9]]
Output: [4, -2, 9]
Explanation: With one row, every column holds a single number, so each column's sum is that number.

Constraints:

• grid has at least 1 row and at most 10 rows.
• Every row has the same number of values, at least 1 and at most 10.
• Every value is an integer between -1000 and 1000.