498. Diagonal Traverse

There are $m+n-1$ diagonals. We traverse each element and append them in forward or reverse order depending on the parity of the diagonal’s index.

class Solution:
    def findDiagonalOrder(self, mat: List[List[int]]) -> List[int]:
        if not mat or not mat[0]:
            return []
 
        m, n = len(mat), len(mat[0])
        result = []
 
        for d in range(m + n - 1):  
            intermediate = []
 
            r = 0 if d < n else d - n + 1
            c = d if d < n else n - 1
 
            while r < m and c >= 0:
                intermediate.append(mat[r][c])
                r += 1
                c -= 1
 
            if d % 2 == 0:
                result.extend(reversed(intermediate))
            else:
                result.extend(intermediate)
 
        return result