Find Inorder Successor of a Binary Search Tree in C++

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

So, if the input is like

And 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)

Let us see the following implementation to get better understanding −

Example

 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

TreeNode *root = new TreeNode(2);
root->left = new TreeNode(1);
root->right = new TreeNode(3);
1

Output

2