【LeetCode 144_二叉树_遍历】Binary Tree Preorder Traversal

解法一：非递归

 1 vector<int> preorderTraversal(TreeNode* root) 

 2 {

 3     vector<int> res;

 4     if (root == NULL)

 5         return res;

 6 

 7     stack<TreeNode*> node_stack;

 8     TreeNode *p = root;

 9     while (p || !node_stack.empty()) {

10         if (p) {

11             res.push_back(p->val);

12             node_stack.push(p);

13             p = p->left;

14         } else {

15             p = node_stack.top();

16             node_stack.pop();

17             p = p->right;

18         }

19     }

20     return res;

21 }

解法二：递归

 1 void preorderTraversalSub(TreeNode* root, vector<int> &res)

 2 {

 3     if (root == NULL)

 4         return;

 5     res.push_back(root->val);

 6     preorderTraversalSub(root->left, res);

 7     preorderTraversalSub(root->right, res);

 8 }

 9 

10 vector<int> preorderTraversal(TreeNode* root)

11 {

12     vector<int> res;

13     preorderTraversalSub(root, res);

14     return res;

15 }