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: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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;
    }