LeetCode 1329. Sort the Matrix Diagonally

原题链接在这里：https://leetcode.com/problems/sort-the-matrix-diagonally/description/

题目：

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].

Given an m x n matrix mat of integers, sort each matrix diagonal in ascending order and return the resulting matrix.

Example 1:

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]]

Example 2:

Input: mat = [[11,25,66,1,69,7],[23,55,17,45,15,52],[75,31,36,44,58,8],[22,27,33,25,68,4],[84,28,14,11,5,50]]
Output: [[5,17,4,1,52,7],[11,11,25,45,8,69],[14,23,25,44,58,15],[22,27,31,36,50,66],[84,28,75,33,55,68]]

Constraints:

• m == mat.length
• n == mat[i].length
• 1 <= m, n <= 100
• 1 <= mat[i][j] <= 100

题解：

For the same diagonal nums, mat[i][j], i - j should be the same.

Group all the numbers based on i - j, sort them and assign them back.

Time Complexity: O(m * n * log(Math.min(m, n)). m = mat.length. n = mat[0].length.

Space: O(m * n).

AC Java:

class Solution {
    public int[][] diagonalSort(int[][] mat) {
        if(mat == null || mat.length == 0 || mat[0].length == 0){
            return mat;
        }
        
        int m = mat.length;
        int n = mat[0].length;
        HashMap<Integer, PriorityQueue<Integer>> hm = new HashMap<>();
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                hm.putIfAbsent(i - j, new PriorityQueue<Integer>());
                hm.get(i - j).add(mat[i][j]);
            }
        }
        
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                mat[i][j] = hm.get(i - j).poll();
            }
        }
        
        return mat;
    }
}