Find Mode in Binary Search Tree LeetCode solution in Java

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class Solution {
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  public int[] findMode(TreeNode root) {
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    if (root == null) return new int[0];
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    Map<Integer, Integer> map =
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        new HashMap<
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            Integer,
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            Integer>(); // we are taking map to count each and every value of the tree and the no of
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    // times they occurs in the tree
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    Queue<TreeNode> qu = new LinkedList<TreeNode>(); // to itereate over the tree
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    List<Integer> list =
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        new ArrayList<
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            Integer>(); // to save our result into a dynamic arraylist then will convert into static
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    // array for return it
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    qu.add(root); // add the first root node into queue to iterate over the tree
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    while (!qu.isEmpty()) {
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      TreeNode tmp = qu.poll(); // we poll out the node which is last inputed to the queue
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      if (map.containsKey(
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          tmp.val)) { // we are checking through the map wheather the value this node have already
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        // stored into the map or not
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        map.put(
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            tmp.val,
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            map.get(tmp.val)
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                + 1); // the value is already stored then we just increase the count by 1
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      } else {
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        map.put(tmp.val, 1); // if the value is unique then we store it to the map with the count 1
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      }
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      if (tmp.left != null)
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        qu.add(
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            tmp.left); // this way we are checking wheather left node has any value or not respect
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      // to the current poped element of queue
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      if (tmp.right != null)
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        qu.add(tmp.right); // the same thing of the above just this is for right node of respective
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      // poped out node
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    }
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    int max =
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        Integer.MIN_VALUE; // we are taking it because of requirement to identify highest no of
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    // repeated node available in this tree
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    for (Integer key :
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        map.keySet()) { // we are using keySet() for iterating over the map here key is differernt
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      // nodes and value is the no of count they have in this tree
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      if (map.get(key)
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          > max) { // if anything we find have greater value then previous maximum no of node like 2
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        // 2 2 - value 3, 3 3 3 3 - value 4 so now 4 is the maximum now
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        list.clear(); // just to clear previous data that are stored into that list
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        max = map.get(key); // now max will replaced by the new no of count of a node
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        list.add(key); // we are adding the key which has most no of count
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      } else if (max
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          == map.get(
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              key)) { // if we found another node which also has present maximum no of node count in
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        // the tree
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        list.add(key); // we are also adding those key
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      }
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    }
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    // now we just create an array transfer hole data that arraylist has and then return
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    int[] res = new int[list.size()];
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    for (int i = 0; i < list.size(); i++) {
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      res[i] = list.get(i);
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    }
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    return res;
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  }
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}