Diagonal Traverse II LeetCode solution in Java

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class Solution {
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  public int[] findDiagonalOrder(List<List<Integer>> nums) {
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    HashMap<Integer, Stack<Integer>> map = new HashMap();
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    for (int i = 0; i < nums.size(); i++) {
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      for (int j = 0; j < nums.get(i).size(); j++) {
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        int z = i + j;
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        if (map.containsKey(z)) {
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          map.get(z).add(nums.get(i).get(j));
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        } else {
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          Stack<Integer> stk = new Stack<>();
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          stk.push(nums.get(i).get(j));
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          map.put(z, stk);
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        }
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      }
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    }
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    ArrayList<Integer> arr = new ArrayList<>();
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    int k = 0;
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    while (true) {
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      if (map.containsKey(k)) {
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        int size = map.get(k).size();
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        while (size-- > 0) {
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          arr.add(map.get(k).pop());
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        }
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        k++;
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      } else {
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        break;
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      }
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    }
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    int[] res = new int[arr.size()];
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    for (int i = 0; i < res.length; i++) {
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      res[i] = arr.get(i);
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    }
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    return res;
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  }
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}