剑指 Offer 32 - II. 从上到下打印二叉树 II

[
[3],
[9,20],
[15,7]
]

要求按层次输出， 所以每次需要记录一下当前层数的队列大小，然后遍历当前层的队列

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * } 
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null) return res;
        List<TreeNode> nodelist = new ArrayList<>();
        nodelist.add(root);
        while(!nodelist.isEmpty()){
            int listlength = nodelist.size();
            List<Integer> templ = new ArrayList<>();
            for(int i =0; i<listlength;i++){
                TreeNode tempn = nodelist.remove(0);
                templ.add(tempn.val);
                if(tempn.left != null) nodelist.add(tempn.left);
                if(tempn.right != null) nodelist.add(tempn.right);
            }
            res.add(templ);
        }
        return res;
    }
}