144、Binary Tree Preorder Traversal

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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?

解题思路：

前序遍历的迭代方式。

c++版本：

class Solution {public:    void visitAlongLeft(TreeNode* x,stack<TreeNode*>&s,vector<int>& res){        while(x){            res.push_back(x->val);            s.push(x->right);            x = x->left;        }    }    vector<int> preorderTraversal(TreeNode* root) {        stack<TreeNode*> s;        vector<int> res;        //TreeNode *x = root;        while(true){            visitAlongLeft(root,s,res);            if(s.empty())                break;            root = s.top();s.pop();        }        return res;    }};

python版本：

class Solution(object):    def preorderTraversal(self, root):        """        :type root: TreeNode        :rtype: List[int]        """        def visitAlongLeft(x,s,res):            while(x!=None):                res.append(x.val)                s.append(x)                 x = x.left        s,res = [], []        while True:            visitAlongLeft(root,s,res)            if(len(s)==0):break            root = s.pop()            root = root.right        return res

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