Find Largest Value in Each Tree Row LeetCode solution in Java

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/**
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 * Definition for a binary tree node. public class TreeNode { int val; TreeNode left; TreeNode
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 * right; TreeNode() {} TreeNode(int val) { this.val = val; } TreeNode(int val, TreeNode left,
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 * TreeNode right) { this.val = val; this.left = left; this.right = right; } }
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 */
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class Solution {
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  private List<Integer> li = new ArrayList<>();
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  public List<Integer> largestValues(TreeNode root) {
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    if (root == null) return li; // if root is NULL
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    // using bfs(level-order)
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    Queue<TreeNode> q = new LinkedList<>();
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    q.add(root);
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    while (!q.isEmpty()) {
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      int size = q.size();
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      int res = Integer.MIN_VALUE;
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      while (size-- > 0) {
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        TreeNode temp = q.poll();
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        if (temp.left != null) q.add(temp.left);
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        if (temp.right != null) q.add(temp.right);
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        res = Math.max(res, temp.val); // comparing every node in each level to get max
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      }
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      li.add(res); // adding each level Max value to the list
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    }
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    return li;
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  }
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}