285. Inorder Successor in BST

It's very easy to miss the case that when we go into left subtree, current root become the successor.

1:  /**  
2:   * Definition for a binary tree node.  
3:   * struct TreeNode {  
4:   *   int val;  
5:   *   TreeNode *left;  
6:   *   TreeNode *right;  
7:   *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
8:   * };  
9:   */  
10:  class Solution {  
11:  public:  
12:    TreeNode* inorderSuccessor(TreeNode* root, TreeNode* p) {  
13:      TreeNode *successor = NULL;  
14:      while (root) {  
15:        if (root->val == p->val) {  
16:          if (root->right == NULL) return successor;  
17:          else {  
18:            root = root->right;  
19:            while (root->left) root = root->left;  
20:            return root;  
21:          }  
22:        } else if (root->val < p->val) {  
23:          root = root->right;  
24:        } else {  
25:          successor = root;  
26:          root = root->left;  
27:        }  
28:      }  
29:      return NULL;  
30:    }  
31:  };