Leetcode 515. 在每个树行中找最大值

题目链接

https://leetcode-cn.com/problems/find-largest-value-in-each-tree-row/description/

题目描述

您需要在二叉树的每一行中找到最大的值。

示例：

输入: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

输出: [1, 3, 9]

题解

按层次遍历二叉树，计算每层的最大值即可。

代码

/**
 * 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> list = new ArrayList<>();
        if (root == null) { return list; }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            int max = Integer.MIN_VALUE;
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.val > max) { max = node.val; }
                if (node.left != null) { queue.offer(node.left); }
                if (node.right != null) { queue.offer(node.right); }
            }
            list.add(max);
        }
        return list;
    }
}