力扣算法题—144Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

Solution：

　　很简单，使用栈，先存入右节点，再存入左节点，这样就是先弹出左节点了

　　

 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode* root) {
         if (root == nullptr)return {};
         vector<int>res;
         stack<TreeNode*>s;
         s.push(root);
         while (!s.empty())
         {
             TreeNode* p = s.top();
             s.pop();
             res.push_back(p->val);
             if (p->right)s.push(p->right);
             if (p->left)s.push(p->left);
         }
         return res;
     }
 };