94.二叉树的中序遍历

一、递归

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new Arraylist <> ();//结果列表
        Stack<TreeNode> stack =new Stack <> (); //栈
        TreeNode cur = root ; //指向当前节点
        while (cur! = null ||!stack.isEmpty() ){
            if (cur !=null) {
                stack.push(cur);
                cur = cur.left;// 指向左子树
            }
        else {
            cur = stack.pop();//当前节点为空说明已经到最左侧 弹出上一个根节点
            list.add(cur.val);//添加到结果list中
            cur = cur.right;//指向右子树 
        }
        }
        return list ;
    }
}

二、遍历

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList <Integer> ();
        inorder (root , res);
        return res ;
    }
    public void inorder(TreeNode root ,List<Integer> res){
        if (root == null){
            return ;
        }
        inorder(root.left , res);
        res.add(root.val);
        inorder(root.right ,res);

    }

}