剑指offer 二叉树的下一个结点

/*
struct TreeLinkNode {
    int val;
    struct TreeLinkNode *left;
    struct TreeLinkNode *right;
    struct TreeLinkNode *next;
    TreeLinkNode(int x) :val(x), left(NULL), right(NULL), next(NULL) {
        
    }
};
*/
class Solution {
public:
    TreeLinkNode* GetNext(TreeLinkNode* pNode){
        if(pNode == nullptr){
            return pNode;
        }        
        //如果有右子树
        if(pNode -> right != nullptr){
            pNode = pNode -> right;
            while(pNode -> left != nullptr){
                pNode = pNode -> left;
            }
            return pNode;
        }
        //如果没有右子树        
        while(pNode -> next != nullptr){//第一个转向右边的结点就是小范围根节点
            if(pNode -> next -> left == pNode){
                return pNode -> next;
            }
            pNode = pNode -> next;
        }
        return nullptr;
    }
};