leecode 94. 二叉树的中序遍历

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);
    }
}

栈法

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root==null) return res;
        Stack<TreeNode> st = new Stack();
        while( !st.isEmpty() || root!=null ) {      //注意停止条件
            while (root != null) {
                st.push(root);
                root = root.left;
            }
            root = st.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }