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].

递归：

/** * 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) {        if(root){values.push_back(root->val);preorderTraversal(root->left);preorderTraversal(root->right);    return values;        }else{    return values;  }    }private:vector<int> values;};

非递归：

/** * 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) {        stack <TreeNode *> paths;while(root || !paths.empty()){while(root){values.push_back(root->val);paths.push(root);root = root->left;}if(!paths.empty()){TreeNode *p=paths.top();paths.pop();root = p->right;}}return values;    }private:vector<int> values;};