LeetCode 144. Binary Tree Preorder Traversal 二叉树的前序遍历 C++

Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [,null,,]

    \

    /

Output: [,,]

Follow up: Recursive solution is trivial, could you do it iteratively?

方法一：使用迭代（C++）

 vector<int> preorderTraversal(TreeNode* root) {
         vector<int> res={};
         if(!root)
             return res;
         stack<TreeNode*> s;
         TreeNode* cur=root;
         while(!s.empty()||cur){
             while(cur){
                 res.push_back(cur->val);
                 s.push(cur);
                 cur=cur->left;
             }
             cur=s.top();
             s.pop();
             cur=cur->right;
         }
         return res;
     }

方法二：使用递归，更简单，但是效率较低

 void preOrder(TreeNode* root,vector<int>& m){
         if(!root)
             return;
         m.push_back(root->val);
         preOrder(root->left,m);
         preOrder(root->right,m);
     }

     vector<int> preorderTraversal(TreeNode* root) {
         vector<int> res={};
         if(!root)
             return res;
         preOrder(root,res);
         return res;
     }