LeetCode 515. Find Largest Value in Each Tree Row

515. Find Largest Value in Each Tree Row

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• Difficulty: Medium
• Contributors: love_FDU_llp

You need to find the largest value in each row of a binary tree.

Example:

Input: 

          1
         / 
        3   2
       /      
      5   3   9 

Output: [1, 3, 9] 

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【题目分析】

给定一颗二叉树，返回二叉树中每一层的最大值。

【思路】

这是一个典型的二叉树层序遍历的问题，需要使用队列来解决。关于java队列的使用，计划详细了解一下。

【java代码】

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