Diagonal Traverse II LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> findDiagonalOrder(vector<vector<int>> &nums) {
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    int n = nums.size();
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    int m = 0;
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    for (vector<int> &v : nums) {
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      int y = v.size();
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      m = max(m, y);
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    }
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    vector<int> D2[n + m - 1];
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    for (int i = 0; i < n; i++) {
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      for (int j = 0; j < nums[i].size(); j++) {
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        D2[i + j].push_back(nums[i][j]);
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      }
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    }
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    vector<int> ans;
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    for (int i = 0; i < n + m - 1; i++) {
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      int x = D2[i].size();
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      while (x--) {
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        ans.push_back(D2[i][x]);
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        D2[i].pop_back();
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      }
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    }
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    return ans;
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  }
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};