144. Binary Tree Preorder Traversal

递归：先序遍历的顺序为根节点，左子树，右子树class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {

vector<int> table;
if(root!=NULL)
preOrder(table,root);
return table;
}
void preOrder(vector<int> &table,TreeNode* root)
{
if(root!=NULL)
table.push_back(root->val); //visit root
if(root->left!=NULL)
preOrder(table,root->left); //visit left
if(root->right!=NULL)
preOrder(table,root->right); //visit right
}
};