<p>Given a 2D integer array <code>nums</code>, return <em>all elements of </em><code>nums</code><em> in diagonal order as shown in the below images</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/04/08/sample_1_1784.png" style="width: 158px; height: 143px;" />
<pre>
<strong>Input:</strong> nums = [[1,2,3],[4,5,6],[7,8,9]]
<strong>Output:</strong> [1,4,2,7,5,3,8,6,9]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/04/08/sample_2_1784.png" style="width: 230px; height: 177px;" />
<pre>
<strong>Input:</strong> nums = [[1,2,3,4,5],[6,7],[8],[9,10,11],[12,13,14,15,16]]
<strong>Output:</strong> [1,6,2,8,7,3,9,4,12,10,5,13,11,14,15,16]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= nums[i].length &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= sum(nums[i].length) &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= nums[i][j] &lt;= 10<sup>5</sup></code></li>
</ul>

#include <vector>
#include <unordered_map>
#include <list>
#include <iostream>
#include <algorithm> // For std::sort

class Solution {
public:
    std::vector<int> findDiagonalOrder(std::vector<std::vector<int>>& nums) {
        std::unordered_map<int, std::list<int>> hashmap;
        
        // Fill the hashmap with diagonals
        for(int i = 0; i < nums.size(); i++) {
            for(int j = 0; j < nums[i].size(); j++) {
                hashmap[i + j].push_front(nums[i][j]);
            }
        }
        
        // Gather the keys and sort them
        std::vector<int> keys;
        for (const auto& pair : hashmap) {
            keys.push_back(pair.first);
        }
        std::sort(keys.begin(), keys.end());

        // Construct the result vector using the sorted keys
        std::vector<int> result;
        for (int key : keys) {
            for (int num : hashmap[key]) {
                result.push_back(num);
            }
        }

        return result;
    }
};

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