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1424. Diagonal Traverse II 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<int> findDiagonalOrder(vector<vector<int>>& nums) {
    vector<int> ans;
    unordered_map<int, vector<int>> keyToNums;  // key := row + column
    int maxKey = 0;

    for (int i = 0; i < nums.size(); ++i)
      for (int j = 0; j < nums[i].size(); ++j) {
        const int key = i + j;
        keyToNums[key].push_back(nums[i][j]);
        maxKey = max(maxKey, key);
      }

    for (int i = 0; i <= maxKey; ++i)
      for (auto it = keyToNums[i].rbegin(); it != keyToNums[i].rend(); ++it)
        ans.push_back(*it);

    return ans;
  }
};

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class Solution {
  public int[] findDiagonalOrder(List<List<Integer>> nums) {
    List<Integer> ans = new ArrayList<>();
    Map<Integer, List<Integer>> keyToNums = new HashMap<>(); // key := row + column
    int maxKey = 0;

    for (int i = 0; i < nums.size(); ++i)
      for (int j = 0; j < nums.get(i).size(); ++j) {
        final int key = i + j;
        keyToNums.putIfAbsent(key, new ArrayList<>());
        keyToNums.get(key).add(nums.get(i).get(j));
        maxKey = Math.max(key, maxKey);
      }

    for (int i = 0; i <= maxKey; ++i)
      for (int j = keyToNums.get(i).size() - 1; j >= 0; --j)
        ans.add(keyToNums.get(i).get(j));

    return ans.stream().mapToInt(Integer::intValue).toArray();
  }
}