找到二叉树每一层最大的数，并输出

找到二叉树每一层最大的数，并输出

/**
 * 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> result = new ArrayList<Integer>();
      if(root ==null){
          return result;
      }

        result.add(root.val);
        LinkedList<TreeNode> temp = new LinkedList<TreeNode>();
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        TreeNode current = null;
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> needSort = new ArrayList<Integer>(); 
            int size = queue.size();
            for(int i=0;i<size;i++){
                current = queue.poll();
                System.out.println(current.val);
                if(current.left!=null){
                    temp.offer(current.left);
                    needSort.add(current.left.val);
                }
                if(current.right!=null){
                    temp.offer(current.right);
                    needSort.add(current.right.val);
                }
            }
            
            if(!needSort.isEmpty()){
                Collections.sort(needSort);
                result.add(needSort.get(needSort.size()-1));
            }
            
            if(!temp.isEmpty()){
                int siz = temp.size();
                for(int j=0;j<siz;j++){
                    queue.offer(temp.poll());
                }
            }
            
        }
        return result;
        
    }
}

层次遍历时，queue pop出当层所有节点后，再加入下一层节点。