LeetCode | Binary Tree Preorder Traversal

题目：

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?

思路：

常规的递归调用PreOrder Traversal

代码：

递归版本

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:vector<int> v;    vector<int> preorderTraversal(TreeNode *root) {        preorder(root);        return v;    }        void preorder(TreeNode *root)    {        if(root !=NULL)        {            v.push_back(root->val);            preorder(root->left);            preorder(root->right);        }    }};

非递归版本

/** * Definition for binary tree * 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> result;        stack<TreeNode*> node;                if(root != NULL){            node.push(root);        }                while(!node.empty()){            TreeNode* p = node.top();            node.pop();                        result.push_back(p->val);            if(p->right != NULL){                node.push(p->right);            }            if(p->left != NULL){                node.push(p->left);            }        }                return result;    }};