在中序遍历中调整： 
static struct TreeNode* q=NULL;
void order(struct TreeNode* p){
    if(p->left)
        order(p->left);
    p->left=q;
    if(q)
        q->right=p;
    q=p;
    if(p->right)
        order(p->right);
}
struct TreeNode* Convert(struct TreeNode* pRootOfTree ) {
    // write code here
    if(!pRootOfTree) 
        return NULL;
    order(pRootOfTree);
    while(pRootOfTree->left) 
        pRootOfTree=pRootOfTree->left;
    return pRootOfTree;
}
存储中序遍历结点再连接： 
class Solution {
public:
    vector<TreeNode*> tree;
    vector<TreeNode*> order(TreeNode* p){
        if(p->left)
            order(p->left);
        tree.push_back(p);
        if(p->right)
            order(p->right);
        return tree;
    }
    TreeNode* Convert(TreeNode* pRootOfTree) {
        if(!pRootOfTree)
            return NULL;
        order(pRootOfTree);
        for(int i=0;i<tree.size()-1;i++){
            tree[i]->right=tree[i+1];
            tree[i+1]->left=tree[i];
        }
        return tree[0];
    }
};