144. 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

 

 递归方案：

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

迭代方案：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode*> stackt;
        if(root==NULL) return result;
        stackt.push(root);
        while(stackt.size()){
            TreeNode* treenode=stackt.top();
            stackt.pop();
            result.push_back(treenode->val);
            if(treenode->right!=NULL){
                stackt.push(treenode->right);
            }
            if(treenode->left!=NULL){
                stackt.push(treenode->left);
            }
        }
        return result;
     
            
    }
};