【LeetCode】Binary Tree Postorder Traversal

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

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [3,2,1].

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


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


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