/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode *father;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL), father(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* inorderSuccessor(TreeNode* p) {
        //分二种情况，1.有右儿子，此时中序中后继为右子树最左边的节点2.无右子树，此时沿着当前节点
        //向上找，直至找到一个节点，此节点的左孩子中包括当前结点，这个节点就是后继
        if(p->right){
            p = p->right;
            while(p->left) p = p->left;
            return p;
        }

        while(p->father && p == p->father->right)   p = p->father;
        return p->father;
    }
};