[LeetCode]Binary Tree Preorder Traversal

题目描述:(链接）

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：

迭代版：

/**
* 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> result;
stack<TreeNode *> cache;
TreeNode *p = root;
if (p != nullptr) {
cache.push(p);
}

while (!cache.empty()) {
p = cache.top();
cache.pop();
result.push_back(p->val);

if (p->right != nullptr) {
cache.push(p->right);
}

if (p->left != nullptr) {
cache.push(p->left);
}
}

return result;
}
};

递归版：

/**
* 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) {
if (root != nullptr) {
result.push_back(root->val);
preorderTraversal(root->left);
preorderTraversal(root->right);
}

return result;
}
private:
vector<int> result;
};