145. Binary Tree Postorder Traversal

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

Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
TreeNode walker = root;
TreeNode lastProcessed = null;
while(walker!=null || !stack.isEmpty())
{
if(walker == null)
{
TreeNode processing = stack.peek();
if(processing.right != null && processing.right != lastProcessed)
{
walker = processing.right;
}
else
{
results.add(processing.val);
lastProcessed = stack.pop();
}
}
else
{
stack.push(walker);
walker = walker.left;
}

}
return results;
}
}