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

解法一：非递归

 vector<int> preorderTraversal(TreeNode* root)
 {
     vector<int> res;
     if (root == NULL)
         return res;

     stack<TreeNode*> node_stack;
     TreeNode *p = root;
     while (p || !node_stack.empty()) {
         if (p) {
             res.push_back(p->val);
             node_stack.push(p);
             p = p->left;
         } else {
             p = node_stack.top();
             node_stack.pop();
             p = p->right;
         }
     }
     return res;
 }

解法二：递归

 void preorderTraversalSub(TreeNode* root, vector<int> &res)
 {
     if (root == NULL)
         return;
     res.push_back(root->val);
     preorderTraversalSub(root->left, res);
     preorderTraversalSub(root->right, res);
 }

 vector<int> preorderTraversal(TreeNode* root)
 {
     vector<int> res;
     preorderTraversalSub(root, res);
     return res;
 }