Leetcode #1329: Sort the Matrix Diagonally
In this guide, we solve Leetcode #1329 Sort the Matrix Diagonally in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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].
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Matrix, Sorting
Intuition
Sorting reveals structure that is hard to see in the original order.
Once sorted, a linear scan is usually enough to compute the answer.
Approach
Sort the data and sweep through it while maintaining a small state.
This keeps the logic straightforward and reliable.
Steps:
- Sort the data.
- Scan in order while maintaining state.
- Update the best answer.
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
The time complexity is , and the space complexity is . The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.