递归

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
     vector<int> ans;
    vector<int> inorderTraversal(TreeNode* root) {
        // write code here
        dfs(root);
        return ans;
    }
    void dfs(TreeNode* root) {
        if (!root) return;
        dfs(root->left);
        ans.push_back(root->val);
        dfs(root->right);
    }
};

迭代

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    vector<int> inorderTraversal(TreeNode* root) {
        // write code here
        vector<int> res;
        stack<TreeNode*> stk;
        while(root || stk.size()) {
            while(root) {
                
                
                
                stk.push(root);
                root = root->left;

            }
            root = stk.top();
            stk.pop();
            res.push_back(root->val);
            root = root->right;
        }
        return res;
    }
};