Solution

/*
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->right) {    // 当前节点有右孩子
            TreeLinkNode* cur = pNode->right;
            while (cur->left) cur = cur->left;
            return cur;
        }
        while (pNode->next) {    // 当前节点没有右孩子
            TreeLinkNode* cur = pNode->next;
            if (cur->left == pNode)    // 当前节点是左孩子结点
                return cur;
            pNode = pNode->next;
        }
        return nullptr;
    }
};