42: Binary Tree Postorder Traversal

  public List<Integer> postorderTraversal(TreeNode root) 
            {
                List<Integer> source=new ArrayList<Integer>();
                postorderTree(root,source);
                return source;
            }
        
        
            private void postorderTree(TreeNode node,List<Integer> nodes)
            {
                    if(node==null)
                    {
                        return;
                    }
                    System.out.println("pre root-->"+node.val);
                    postorderTree(node.left,nodes);
                    postorderTree(node.right,nodes);
                    nodes.add(node.val);
                    System.out.println("-->"+node.val);
            } 
            
            /****后序遍历 PostOrder tree  Time: O(n), Space: O(n) 栈迭代方法实现的********************************************************************/
            //实现的有点繁琐好像,有没有其他办法呢？ 改进版本见 ：postorderTraversal3
            public List<Integer>postorderTraversal2(TreeNode root)
            {
                List<Integer> results=new ArrayList<Integer>();
                Stack<Map<TreeNode, Boolean> > stack=new Stack<Map<TreeNode, Boolean> >();
                TreeNode node=null;
                node=root;
                while(node!=null||!stack.isEmpty())
                {
                    if(node!=null)
                    {
                        Map<TreeNode, Boolean> temp=new HashMap<TreeNode, Boolean>();
                        temp.put(node, false);
                        stack.push(temp);
                        node=node.left;
                    }
                    else
                    {
                        Map<TreeNode, Boolean> tempnode= stack.pop();
                        for (Map.Entry<TreeNode, Boolean> entry :tempnode.entrySet())
                        {
                            if(entry.getKey().left==null&&entry.getKey().right==null&&entry.getValue()==false) // 弹出叶子节点,按照左节点到右节点的弹出顺序
                            {
                                results.add(entry.getKey().val);
                                //System.out.println(entry.getKey().val);
                                node=entry.getKey();
                                node=node.right;
                            }
                            else if(entry.getKey().right!=null&&entry.getValue()==false)// 中间路径上的根节点第一遍时不弹出，设置已访问过
                            {
                                entry.setValue(true);
                                stack.push(tempnode);
                                node=entry.getKey();
                                node=node.right;
                            }
                        else   // 中间路径上的根节点第二遍时弹出
                            {
                                results.add(entry.getKey().val);
                                //System.out.println(entry.getKey().val);
                                node=null;
                            }
                        }
                    }
                }
                return results;
                
            }
            
            /****后序遍历 PostOrder tree 简洁版本1  Time: O(n), Space: O(n) 栈迭代方法实现的********************************************************************/
            /*
             *
             * 算法思路： 对应函数：  postorderTraversal3，postorderTraversal4
             * 
             * pre-order traversal is root-left-right, and post order is left-right-root. 
             * modify the code for pre-order to make it root-right-left, 
             * and then reverse the output so that we can get left-right-root .

    Create an empty stack, Push root node to the stack.

    Do following while stack is not empty.

    2.1. pop an item from the stack and print it.

    2.2. push the left child of popped item to stack.

    2.3. push the right child of popped item to stack.

    reverse the ouput.

             * */
            public List<Integer>postorderTraversal3(TreeNode root)
            {
                List<Integer> results=new ArrayList<Integer>();
                Stack<TreeNode> stack=new Stack<TreeNode>();
                TreeNode node=null;
                node=root;
                while(node!=null||!stack.isEmpty())
                {
                    if(node!=null)
                    {
                        //System.out.println("-->"+node.val);
                        results.add(node.val);
                        stack.push(node);
                        node=node.right;
                    }
                    else
                    {
                        node= stack.pop();
                        node=node.left;
                    }
                }
                Collections.reverse(results);
                return results;
            }
            
            
            public List<Integer>postorderTraversal4(TreeNode root)
            {
                List<Integer> results=new ArrayList<Integer>();
                Stack<TreeNode> stack=new Stack<TreeNode>();
                stack.push(root);
                TreeNode node=null;
                while(!stack.isEmpty())
                {
                    node=stack.pop();
                    results.add(node.val);
                    if(node.left!=null)
                    {
                        stack.push(node.left);
                    }
                    if(node.right!=null)
                    {
                        stack.push(node.right);
                    }
                }
                Collections.reverse(results);
                return results;
            }