see-144. Binary Tree Preorder Traversal

递归：先序遍历的顺序为根节点，左子树，右子树

/**
 * 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> table;
        if(root!=NULL)
            preOrder(table,root);
        return table;
    }
    void preOrder(vector<int> &ans,TreeNode *root)
    {
        stack<TreeNode*> rootTemp;
        TreeNode *p=root;
        while(p!=NULL||rootTemp.size()!=0)
        {
            if(p!=NULL)
            {
                rootTemp.push(p);
                ans.push_back(p->val);
                p=p->left;
            }
            else
            {
                p=rootTemp.top();
                rootTemp.pop();
                p=p->right;
            }
        }
    }
    /*
    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
    }
    */
};