[자료구조] Binary Tree Postorder Traversal

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Given a binary tree, return the postorder traversal of its nodes’ values.

For example:
Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

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