Inorder Successor in BST in C++

Suppose we have a binary search tree and a node in it, we have to search the in-order successor of that node in the BST. As we know that the successor of a node p is the node with the smallest key greater than p.val.

So, if the input is like root = [2,1,3], p = 1,

then the output will be 2,

To solve this, we will follow these steps −

• Define recursive method inorderSuccessor(), this will take root and p

• if root null, then −

  • return null

• if val of root <= val of p, then −

  • return inorderSuccessor(right of root , p)

• Otherwise

  • option := inorderSuccessor(left of root , p)

  • return (if option is zero, then root, otherwise option)

Example 

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class TreeNode{
public:
   int val;
   TreeNode *left, *right;
   TreeNode(int data){
      val = data;
      left = NULL;
      right = NULL;
   }
};
class Solution {
public:
   TreeNode* inorderSuccessor(TreeNode* root, TreeNode* p) {
      if(!root) return NULL;
      if(root->val <= p->val){
         return inorderSuccessor(root->right, p);
      }
      else{
         TreeNode* option = inorderSuccessor(root->left, p);
         return !option ? root : option;
      }
   }
};
main(){
   TreeNode *root = new TreeNode(2);
   root->left = new TreeNode(1);
   root->right = new TreeNode(3);
   TreeNode *p = root->left;
   Solution ob;
   cout << (ob.inorderSuccessor(root, p))->val;
}

Input

{2,1,3},1

Output

2