An anti-diagonal of a grid runs from the top-right toward the bottom-left. Given a grid grid with
R rows and C columns, return a list of length R + C - 1 whose entry k is the sum of all cells
grid[r][c] with r + c == k.
Examples
Input: grid = [[1, 2, 3],
[4, 5, 6]]
Output: [1, 6, 8, 6]
Explanation: k=0: 1; k=1: 2+4; k=2: 3+5; k=3: 6.
Input: grid = [[1, 2],
[3, 4]]
Output: [1, 5, 4]
Constraints
0 <= rows, cols <= 100; an empty grid gives[]
Goals
- Recognise that cells on one anti-diagonal share the value r + c
- Accumulate into a list indexed by that value