Diagonal Traverse 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>> &mat) {
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    vector<int> vec;
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    int m = mat.size() - 1, n = mat[0].size() - 1;
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    int i = 0, j = 0;
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    if (m == -1) {
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      return {};
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    }
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    if (m == 0) {
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      for (int i{0}; i <= n; ++i) {
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        vec.push_back(mat.at(0).at(i));
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      }
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      return vec;
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    } else if (n == 0) {
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      for (int i{0}; i <= m; ++i) {
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        vec.push_back(mat.at(i).at(0));
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      }
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      return vec;
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    }
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    for (int k{0}; k <= m + n; k++) {
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      if (k == 0) {
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        vec.push_back(mat.at(0).at(0));
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        j++;
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      } else if (k == m + n) {
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        vec.push_back(mat.at(m).at(n));
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      } else if (k % 2 != 0) {
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        while (i <= m && j >= 0 && j <= n) {
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          vec.push_back(mat.at(i).at(j));
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          i++;
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          j--;
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        }
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        if (i <= m && j < 0) {
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          j++;
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        } else if (i > m && j >= 0) {
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          j += 2;
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          i--;
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        } else if (i > m && j < 0) {
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          i--;
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          j += 2;
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        }
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      } else {
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        while (i >= 0 && j <= n) {
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          vec.push_back(mat.at(i).at(j));
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          i--;
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          j++;
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        }
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        if (i < 0 && j <= n) {
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          i++;
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        } else if (i < 0 && j > n) {
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          i += 2;
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          j--;
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        } else if (i > m && j < 0) {
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          i--;
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          j += 2;
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        } else if (i >= 0 && j > n) {
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          i += 2;
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          j--;
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        }
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      }
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    }
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    return vec;
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  }
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};