【leetcode】515. Find Largest Value in Each Tree Row

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]

題解如下：

/**
 * 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> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        if(root == null)
            return res;
        res.add(root.val);
        if(root.left != null)
            queue.add(root.left);
        if(root.right != null)
            queue.add(root.right);
        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.left != null) queue.add(node.left);
                if(node.right != null) queue.add(node.right);
                max = Math.max(max,node.val);
            }
            res.add(max);
        }
        return res;
    }
}