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Sort a 2D vector diagonally using Map Data Structure
  • Last Updated : 21 May, 2021

Given a 2D vector mat[][] of integers. The task is to sort the elements of the vectors diagonally from top-left to bottom-right in increasing order.
Examples:
 

Input: mat[][] = 
{{9, 4, 2}, 
 {7, 4, 6},
 {2, 3, 3}}     
Output: 
3 4 2
3 4 6
2 7 9
Explanation:
There are 5 diagonals in this matrix:
1. {2} - No need to sort
2. {7, 3} - Sort - {3, 7}
3. {9, 4, 3} - Sort - {3, 4, 9}
4. {4, 6} - Already sorted
5. {2} - No need to sort



Input: mat[][] =  
{{ 4, 3, 2, 1 }, 
 { 3, 2, 1, 0 }, 
 { 2, 1, 1, 0 }, 
 { 0, 1, 2, 3 }}
Output: 
1 0 0 1 
1 2 1 2 
1 2 3 3 
0 2 3 4 

 

Approach: 
 

  1. All elements in the same diagonal have the same index difference i – j where i is the row number and j is the column number. So we can use a map to store every diagonal at index i – j. 
     
  2. Now we can sort every index of the map using the inbuilt function. 
     
  3. Now in the original matrix, we can insert every diagonal of a matrix stored in map. 
     
  4. Finally, we can print the Matrix. 
     

Below is the implementation of the above approach:
 

CPP




// C++ implementation to sort the
// diagonals of the matrix
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to sort the
// diagonal of the matrix
void SortDiagonal(int mat[4][4],
                  int m, int n)
{
    // Map to store every diagonal
    // in different indices here
    // elements of same diagonal
    // will be stored in same index
    unordered_map<int, vector<int> > mp;
 
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j < n; j++)
        {
            // Storing diagonal elements
            // in map
            mp[i - j].push_back(mat[i][j]);
        }
    }
 
    // To sort each diagonal in
    // ascending order
    for (int k = -(n - 1); k < m; k++)
    {
        sort(mp[k].begin(),
             mp[k].end());
    }
 
    // Loop to store every diagonal
    // in ascending order
    for (int i = m - 1; i >= 0; i--)
    {
        for (int j = n - 1; j >= 0; j--)
        {
            mat[i][j] = mp[i - j].back();
            mp[i - j].pop_back();
        }
    }
 
    // Loop to print the matrix
    for (int i = 0; i < m; i++) {
        for (int j = 0; j < n; j++)
            cout << mat[i][j] << " ";
        cout << endl;
    }
}
 
// Driven Code
int main()
{
    int arr[4][4] = { { 4, 3, 2, 1 },
                    { 3, 2, 1, 0 },
                    { 2, 1, 1, 0 },
                    { 0, 1, 2, 3 } };
 
    // Sort the Diagonals
    SortDiagonal(arr, 4, 4);
 
    return 0;
}

Javascript




<script>
// Javascript implementation of the above approach
 
// Function to sort the
// diagonal of the matrix
function SortDiagonal(mat, m, n)
{
    // Map to store every diagonal
    // in different indices here
    // elements of same diagonal
    // will be stored in same index
    var mp = {};
    for (var i = 0; i < m; i++)
    {
        for (var j = 0; j < n; j++)
        {
            mp[i - j] = [];
        }
     }
 
    for (var i = 0; i < m; i++)
    {
        for (var j = 0; j < n; j++)
        {
            // Storing diagonal elements
            // in map
            mp[i - j].push(mat[i][j]);
        }
    }
 
    // To sort each diagonal in
    // ascending order
    for (var k = -(n - 1); k < m; k++)
    {
        mp[k].sort();
    }
 
    // Loop to store every diagonal
    // in ascending order
    for (var i = m - 1; i >= 0; i--)
    {
        for (var j = n - 1; j >= 0; j--)
        {
            mat[i][j] = mp[i - j].pop();
        }
    }
 
    // Loop to print the matrix
    for (var i = 0; i < m; i++) {
        for (var j = 0; j < n; j++)
            document.write(mat[i][j] + " " );
        document.write("<br>");
    }
}
 
 
// Driver Code
var arr = [[ 4, 3, 2, 1 ],
    [ 3, 2, 1, 0 ],
    [ 2, 1, 1, 0 ],
    [ 0, 1, 2, 3 ]];
 
// Sort the Diagonals
SortDiagonal(arr, 4, 4);
</script>
Output: 
1 0 0 1 
1 2 1 2 
1 2 3 3 
0 2 3 4 

 

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