力扣-144题二叉树的前序遍历（C++）- 递归+迭代

矫情吗；* 2022-08-29 12:51 147阅读 0赞

题目链接：[https://leetcode-cn.com/problems/binary-tree-preorder-traversal/][https_leetcode-cn.com_problems_binary-tree-preorder-traversal]  
题目如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwNDY3Njcw_size_16_color_FFFFFF_t_70]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwNDY3Njcw_size_16_color_FFFFFF_t_70 1]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzQwNDY3Njcw_size_16_color_FFFFFF_t_70 2]  
思路一：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 {
        
    public:
    
        void pre_order(TreeNode* root,vector<int>& result){
        
            if(root==NULL) return ;
    
            result.push_back(root->val);
            pre_order(root->left,result);
            pre_order(root->right,result);
        }
    
        vector<int> preorderTraversal(TreeNode* root) {
        
            vector<int> result;
    
            pre_order(root,result);
    
            return result;
        }
    };

思路二：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 {
        
    public:
    
        vector<int> preorderTraversal(TreeNode* root) {
        
            //栈可实现递归(迭代法)
            //前序遍历顺序：根左右
            //在栈中，右左，弹出顺序：左右
            vector<int> result;
            stack<TreeNode*> stk;
    
            if(root==NULL) return result;
            stk.push(root);
    
            while(!stk.empty()){
        
                TreeNode* node=stk.top();//根
                stk.pop();
                result.push_back(node->val);
    
                if(node->right!=NULL) stk.push(node->right);//右
                if(node->left!=NULL) stk.push(node->left);//左
            }
    
            return result;
            
        }
    };

[https_leetcode-cn.com_problems_binary-tree-preorder-traversal]: https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
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