<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>

static int io_opt = []() {
  std::ios::sync_with_stdio(false);
  return 0;
}();

class Solution {
 public:
  vector<int> findDiagonalOrder(vector<vector<int>>& nums) {
    vector<int> diagonal_count(1, 0);
    int rsize = static_cast<int>(nums.size());
    for (int i = 0; i < rsize; ++i) {
      for (int j = 0; j < nums[i].size(); ++j) {
        if (diagonal_count.size() < i + j + 2) diagonal_count.resize(i + j + 2);
        ++diagonal_count[i + j + 1];
      }
    }
    for (int i = 1; i < diagonal_count.size(); ++i) {
      diagonal_count[i] += diagonal_count[i - 1];
    }
    vector<int> ans(diagonal_count.back());
    for (int i = rsize - 1; i >= 0; --i) {
      for (int j = 0; j < nums[i].size(); ++j) {
        ans[diagonal_count[i + j]++] = nums[i][j];
      }
    }

    return ans;
  }
};

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