Binary Tree Preorder Traversal

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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].

/** * 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> vec;        if(root == NULL)        {            return vec;        }            stack<TreeNode*> st;        TreeNode *tmp = root;        st.push(tmp);        vec.push_back(tmp -> val);        while(!st.empty())        {                        if(tmp -> left == NULL && tmp -> right == NULL)            {                st.pop();                if(st.empty())                {                    break;                }            }            tmp = st.top();            if(tmp -> left != NULL)            {                TreeNode *t = tmp;                tmp = tmp -> left;                vec.push_back(tmp -> val);                st.push(tmp);                t -> left = NULL;                continue;            }            // if(st.empty())            // {            //     break;            // }            tmp = st.top();            if(tmp -> right != NULL)            {                TreeNode *t = tmp;                tmp = tmp -> right;                vec.push_back(tmp -> val);                st.push(tmp);                t -> right = NULL;                continue;            }        }        return vec;    }};

参考网上

/** * 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> ivec;        if(root == NULL)        {            return ivec;        }        stack<TreeNode*> st;        st.push(root);        while(!st.empty())        {            TreeNode* tmp = st.top();            ivec.push_back(tmp -> val);            st.pop();            if(tmp -> right != NULL)            {                st.push(tmp -> right);            }            if(tmp -> left != NULL)            {                st.push(tmp -> left);            }        }        return ivec;    }};

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