递归

class Solution {
public:
    void inorder(TreeNode *root, vector<int>& res) {
        if (root != nullptr) {
            inorder(root->left, res);
            res.push_back(root->val);
            inorder(root->right, res);
        }
    }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        inorder(root, res);
        return res;
    }
};

迭代

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> st;
        while (root || !st.empty()) {
            if (root) {
                st.push(root);
                root = root->left;
            } else {
                root = st.top();
                st.pop();
                res.push_back(root->val);
                root = root->right;
            }
        }
        return res;
    }
};

• 以前的笔记