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