leetcode 二叉树前序遍历的递归和非递归实现

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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;        preOrder(root, vec);        return vec;    }    //递归写法    /*    void preOrder(TreeNode *root, vector<int> &path)    {        if (root)        {            path.push_back(root->val);            preOrder(root->left, path);            preOrder(root->right, path);        }    }    */    //非递归写法    void preOrder(TreeNode *root, vector<int> &path)    {        stack<TreeNode *> TreeNodeStack;                while (root != NULL || !TreeNodeStack.empty())        {            while (root != NULL)            {                path.push_back(root->val);                TreeNodeStack.push(root);                root = root->left;            }            if (!TreeNodeStack.empty())            {                root = TreeNodeStack.top();                TreeNodeStack.pop();                root = root->right;            }        }    }};

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