1.将树的前序和中序遍历进行了格式上的统一，方便大家的记忆
2.树的后序遍历是左右根，是根右左的逆序，树的先序遍历是根左右，改成树的遍历是根右左即可，再翻转

树的前序遍历

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> res=new LinkedList<>();
    TreeNode p=root;
    Stack<TreeNode> stack=new Stack<>();

    while(!stack.isEmpty()|| p!=null)
    {
        while(p!=null)
        {
            stack.push(p);
            res.add(p.val);
            p=p.left;
        }
        if(!stack.isEmpty())
        {
            p=stack.pop();
            p=p.right;
        }
    }
        return res;
    }

树的中序遍历

public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res=new LinkedList<>();
        Stack<TreeNode> stack=new Stack<>();
        TreeNode p=root;
        while(!stack.isEmpty()|| p!=null)
        {
            while(p!=null)
            {
                stack.push(p);
                p=p.left;
            }
            if(!stack.isEmpty())
            {
                p=stack.pop();
                res.add(p.val);
                p=p.right;
            }
        }
        return res;
    }

后序遍历

public void reverse(List<Integer> l)
    {
        for(int i=0;i<l.size()/2;i++)
        {
            int temp=l.get(i);
            l.set(i,l.get(l.size()-1-i));
            l.set(l.size()-1-i,temp);
        }
    }
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res=new LinkedList<>();
        if(root==null)
            return res;
        TreeNode p=root;
        Stack<TreeNode> stack=new Stack<>();

        while(!stack.isEmpty()|| p!=null)
        {
            while(p!=null)
            {
                stack.push(p);
                res.add(p.val);
                p=p.right;
            }

            if(!stack.isEmpty())
            {
                p=stack.pop();
                p=p.left;
            }
        }

        reverse(res);
        return res;
    }