Leetcode144. Binary Tree Preorder Traversal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？

递归：

class Solution {
public:
    vector<int> res;
    vector<int> preorderTraversal(TreeNode* root) 
    {
        GetAns(root);
        return res;
    }

    void GetAns(TreeNode* root)
    {
        if(root == NULL)
            return;
        res.push_back(root ->val);
        GetAns(root ->left);
        GetAns(root ->right);
    }
};

迭代：

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) 
    {
        vector<int> res;
        if(root == NULL)
            return res;
        stack<TreeNode*> s;
        TreeNode *cur = root;
        while(!s.empty() || cur)
        {
            if(cur != NULL)
            {
                res.push_back(cur ->val);
                if(cur ->right != NULL)
                    s.push(cur ->right);
                cur = cur ->left;
            }
            else
            {
                cur = s.top();
                s.pop();
            }
        }
        return res;
    }
};