题目链接

层序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> list = new ArrayList();
        Deque<TreeNode> queue = new LinkedList<TreeNode>();
        if(root != null){
            queue.offer(root);
        }
        while(!queue.isEmpty()){
            int size = queue.size();
            // Java最小值整型常量表示方法
            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;
    }
}