# C++ Program to Find Lowest Common Ancestor in a Binary Search Tree

A binary tree which has at most two children, specified as left child and right child. This is a C++ Program to find the lowest common ancestor in a Binary Tree.

## Algorithm

Begin Create a structure n to declare data d, a left child pointer l and a right child pointer r.
Create a function to create newnode. Call a function LCA() to Find lowest common ancestor in a binary tree:
Assume node n1 and n2 present in the tree.
If root is null, then return.
If root is not null there are two cases.
a) If both n1 and n2 are smaller than root, then LCA lies in left.
b) If both n1 and n2 are greater than root, then LCA lies in right.
End.

## Example Code

Live Demo

#include<iostream>
using namespace std;
struct n {
int d;
struct n* l, *r;
}*p = NULL;
struct n* newnode(int d) {
p = new n;
p->d= d;
p->l = p->r = NULL;
return(p);
}
struct n *LCA(struct n* root, int n1, int n2) {
if (root == NULL)
return NULL;
if (root->d > n1 && root->d > n2)
return LCA(root->l, n1, n2);
if (root->d< n1 && root->d < n2)
return LCA(root->r, n1, n2);
return root;
}
int main() {
n* root = newnode(9);
root->l = newnode(7);
root->r = newnode(10);
root->l->l = newnode(6);
root->r->l= newnode(8);
root->r->r = newnode(19);
root->r->l->r = newnode(4);
root->r->r->r = newnode(20);
int n1 = 20, n2 = 4;
struct n *t = LCA(root, n1, n2);
cout<<"Lowest Common Ancestor of 20 and 4 is:" <<t->d<<endl;
n1 = 7, n2 = 6;
t = LCA(root, n1, n2);
cout<<"Lowest Common Ancestor of 7 and 6 is:" << t->d<<endl;
}

## Output

Lowest Common Ancestor of 20 and 4 is:9
Lowest Common Ancestor of 7 and 6 is:7 