leetcode--515. Find Largest Value in Each Tree Row

Input: 

          1
         /         3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int curLevelNum = 1;
        while (!queue.isEmpty()) {
            int nextLevelNum = 0;
            int max = Integer.MIN_VALUE;
            while (curLevelNum-- > 0) {
                TreeNode top = queue.poll();
                max = Math.max(max, top.val);
                if (top.left != null) {
                    queue.offer(top.left);
                    nextLevelNum++;
                }
                if (top.right != null) {
                    queue.offer(top.right);
                    nextLevelNum++;
                }
            }
            result.add(max);
            curLevelNum = nextLevelNum;
        }
        return result;
    }
}