(Java) LeetCode 515. Find Largest Value in Each Tree Row —— 在每个树行中找最大值

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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基础中的基础，树的层次遍历/广度优先搜索。在搜索中存储每一行的最大值只不过是多加了两行代码。不应该是中等难度的题。

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Java

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