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?
题目解析:
方案一:
递归方法
class Solution {
public:
vector<int> preorderTraversal(TreeNode *root) {
if(root == NULL)
return res;
res.push_back(root->val);
preorderTraversal(root->left);
preorderTra......
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