LeetCode之Binary Tree Inorder Traversal

 /*方法一：递归中序遍历。*/
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        inorder(root, res);
        return res;
    }
    
    void inorder(TreeNode* root, vector<int> &res){
        if(root == nullptr) return;
        if(root->left != nullptr) inorder(root->left, res);
        res.push_back(root->val);
        if(root->right != nullptr) inorder(root->right, res);
    }
};

 /*方法二：利用栈，迭代法中序遍历*/
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(root == nullptr) return res;
        stack<TreeNode*> s;
        TreeNode *p = root;
        while(!s.empty() || p != nullptr){
            if(p != nullptr){
                s.push(p);
                p = p->left;
            }
            else{
                p = s.top();
                s.pop();
                res.push_back(p->val);
                p = p->right;
            }
        }
        return res;
    }
};