雖然過了，但是還是不是很清楚，中序遍歷，訪問每個非空節點時，將左指標置空，pre記錄每個父節點，父節點的right置為當前節點

/**
 * 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:
    TreeNode* convertBiNode(TreeNode* root) {
        TreeNode
 
*p = root,*pre = NULL,*newroot;
        stack<TreeNode*>s;
        bool flag = true;
        while(p||!s.empty()){
            if(p){
                s.push(p);
                p = p->left;
            }else{
                p = s.top();
                if(flag&&p->left==NULL){
                    newroot 
 
= p;
                    flag = false;
                }
                p->left = NULL;
                if(pre)pre->right = p;
                cout<<p<<" "<<p->left<<" "<<p->right<<endl;
                s.pop();
                pre = p;
                p 
 
= p->right;
            }
        }
        return newroot;
    }
};