Sort the Matrix Diagonally — LeetCode 1329 Python Solution

The problem

A matrix diagonal is a diagonal line of cells starting from some cell in either the topmost row or leftmost column and going in the bottom-right direction until reaching the matrix's end. For example, the matrix diagonal starting from mat[2][0], where mat is a 6 x 3 matrix, includes cells mat[2][0], mat[3][1], and mat[4][2].

Example

Input

mat = [[3,3,1,1],[2,2,1,2],[1,1,1,2]]

Output

[[1,1,1,1],[1,2,2,2],[1,2,3,3]]

Python solution

class Solution:
    def diagonalSort(self, mat: List[List[int]]) -> List[List[int]]:
        m, n = len(mat), len(mat[0])
        g = [[] for _ in range(m + n)]
        for i, row in enumerate(mat):
            for j, x in enumerate(row):
                g[m - i + j].append(x)
        for e in g:
            e.sort(reverse=True)
        for i in range(m):
            for j in range(n):
                mat[i][j] = g[m - i + j].pop()
        return mat

Complexity

Pattern: Matrix and Grid

Treat a 2-D grid as a graph whose neighbours are the four adjacent cells. LeetCode 1329. Sort the Matrix Diagonally is filed here because LeetCode tags it Matrix, which is the vocabulary this hub collects.

The matrix and grid guide has the Python template for the pattern and the 216 LeetCode problems that use it.

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