leetcood学习笔记-102-二叉树的层次遍历

题目描述：

方法一;

class Solution(object):
    def levelOrder(self, root):        if not root:
            return []
        ans = []
        stack = [root]
        while stack:
            tem_stack = []
            tem_ans = []
            for i in stack:
                tem_ans.append(i.val)
                if i.left:
                    tem_stack.append(i.left)
                if i.right:
                    tem_stack.append(i.right)
            stack = tem_stack
            ans.append(tem_ans)
        return ans

 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<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<> ();
        if(root == null){
            return ans;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            while(size > 0){
                TreeNode curr = queue.poll();
                list.add(curr.val);
                size--;
                if (curr.left != null){
                    queue.offer(curr.left);
                }
                if (curr.right != null){
                    queue.offer(curr.right);
                }
            }
            ans.add(list);
        }
        return ans;
    }
}