/**
 * 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> returnArray;
        if(!root)   
        	return returnArray;
        	
        stack<struct TreeNode*> nodeStack;
        struct TreeNode* p = NULL;
        
        nodeStack.push(root);
        while(!nodeStack.empty())
        {
    
    
            p = nodeStack.top();
            returnArray.push_back(p->val);
            nodeStack.pop();
            if(p->right)
                nodeStack.push(p->right);   // 先入栈右节点，再入栈左节点，这样处理栈顶元素的时候先处理左节点
            if(p->left)
                nodeStack.push(p->left);
        }
        return returnArray;
    } 
};