LeetCode144. 二叉树的前序遍历（Binary Tree Preorder Traversal）

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题目描述

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？

递归代码：

/**
 * 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> result;
    vector<int> preorderTraversal(TreeNode* root) {
        if(root){
            result.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return result; 
    }
};

非递归代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
/**
 * 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;
        stack<TreeNode*> s;
        //TreeNode *p=root;
        while(root||!s.empty()){
            if(root){
                s.push(root);
                res.push_back(root->val);
                root=root->left;
            }
            else{
                root=s.top();
                s.pop();
                root=root->right;
            }  
        }
        return res;    
    }
};