Leetcode 144. 二叉树的前序遍历

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？

//前序递归
class Solution {
public:
    vector<int> res;
    vector<int> preorderTraversal(TreeNode* root) {
        pre(root);
        return res;
    }
    void pre(TreeNode *root)
    {
        if (root == nullptr) return;
        res.push_back(root->val);
        pre(root->left);
        pre(root->right);
    }
};

/Morris Traversal算法
/**
 * 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) {
        vector<int> res;
        while (root)
        {
            if (!root->left)
            {
                res.push_back(root->val);
                root = root->right;
            }
            else
            {
                TreeNode *pre = root->left;
                while (pre->right && pre->right != root) pre = pre->right;
                if (pre->right) pre->right = 0, root = root->right;
                else
                {
                    pre->right = root;
                    res.push_back(root->val);
                    root = root->left;
                }
            }
        }
        return res;
    }
};