/**
 * 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> result;
    vector<int> PReorderTraversal(TreeNode* root) {
        transverse(root);
        return result;
    }
    void transverse(TreeNode*node)
    {
        if (node != NULL)
        {
            result.push_back(node->val);
            if(node->left!=NULL)
            {
                transverse(node->left);
            }
            if(node->right!=NULL)
            {
                transverse(node->right);
            }
        }
    }
};

栈S;
p= root;
while(p || S不空){
    while(p){
        访问p节点；
        p的右子树入S;
        p = p的左子树;
    }
    p = S栈顶弹出;
}

/**
 * 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> result;
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*>S;//用来存放右子树的节点
        TreeNode*Rt = root;
        while(Rt||S.size()) //根节点不为空  或者 栈中有元素
        {
            while(Rt) //根节点不为空
            {
            result.push_back(Rt->val);//先将Rt节点的访问值保存
            S.push(Rt->right); //先保存右子树
            Rt = Rt->left;//将指针指向Rt的左子树节点
            }
            Rt = S.top();S.pop(); //当左子树访问完后，紧接着访问右子树，栈顶元素出栈
        }
        return result;
    }
};