LeetCode 144. 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

 示例:

输入: [1,null,2,3]
1

2
/
3

输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/binary-tree-preorder-traversal

递归：

/**
 * 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:
   
    void process(TreeNode* root,vector<int> &ans)
    {
        ans.push_back(root->val);
        if(root->left) process(root->left,ans);
        if(root->right) process(root->right,ans);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        if(root)
            process(root,ans);
        return ans;
    }


};

迭代：

/**
 * 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> ans;
        if(!root) return ans;
        stack<TreeNode*> nodeStack;
        nodeStack.push(root);
        while(!nodeStack.empty()) {
            TreeNode* node = nodeStack.top();
            ans.push_back(node->val);
            nodeStack.pop();
            if(node->right) nodeStack.push(node->right);
            if(node->left) nodeStack.push(node->left);
        }
        return ans;
    }


};