leecode刷题（30）-- 二叉树的后序遍历

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [3,2,1]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> list = new ArrayList();
    public List<Integer> postorderTraversal(TreeNode root) {
        if (root != null) {
            postorderTraversal(root.left);
            postorderTraversal(root.right);
            list.add(root.val);
        }
        return list;
    }
}

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        if root is not None:
            res = res + self.postorderTraversal(root.left)
            res = res + self.postorderTraversal(root.right)
            res = res + [root.val]      
        return res