Leetcode_144_Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?
题意：给一棵二叉树，输出先序遍历
思路：先访问根节点，然后再将其放入栈。
坑：暂无。

/** * 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;
        GetAns(ans, root);
        return ans;
    }

    void GetAns(vector<int> &ans, TreeNode* root)
    {
        stack<TreeNode*> tree;
        while(root!=NULL||!tree.empty())
        {
            if(root == NULL)
            {
                root = tree.top();
                tree.pop();
                root = root->right;
            }
            else
            {
                ans.push_back(root->val);
                tree.push(root);
                root = root->left;
            }
        }
    }
};