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) {
         // 后序遍历
 
        List<Integer> res=new ArrayList<Integer>();
        TreeNode temp=root;
        Stack<TreeNode> st=new Stack<TreeNode>();
        TreeNode tail=null;
        while(temp!=null||!st.isEmpty())
        {
            if(temp!=null)
            {
                st.push(temp);
                temp=temp.left;
            }
            else
            {
                temp=st.pop();
                if(temp.right!=null)
                {
                    if(tail==temp.right)
                    {
                        //访问该节点
                        res.add(temp.val);
                        tail=temp;
                        temp=null;
                    }
                    else
                    {
                        st.push(temp);
                        temp=temp.right;
                    }
                }
                else
                {
                    
                    res.add(temp.val);
                    tail=temp;
                    temp=null;
                }
                
            }
            
            
        }
        return res;
    }
}