144. Binary Tree Preorder Traversal

基础练习：

递归：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    vector<int> res;
public:
    vector<int> preorderTraversal(TreeNode* root) {
        if(root==nullptr) return {};
        res.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);
        return res;
    }
};

迭代：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    vector<int> res;
public:
    vector<int> preorderTraversal(TreeNode* root) {
        if(root==nullptr) return {};
        stack<TreeNode*> st;
        st.emplace(root);
        while(!st.empty()){
            TreeNode* temp =st.top();  
            st.pop();  
            res.emplace_back(temp->val); ////前序遍历就是一开始就把当前节点输出，所以直接加入res数组
            if(temp->right) st.emplace(temp->right);
            if(temp->left) st.emplace(temp->left);   //因为是栈，又要先遍历左子树，所以要后加左节点
        }
        return res;
    }
};