In graph theory and computer science, the lowest common ancestor (LCA) of two nodes is the lowest node that has both of them as descendants.

Here is an example, given a Binary Search Tree:

```
LCA(2, 8) = 6 LCA(4, 7) = 6LCA(2, 5) = 2LCA(0, 3) = 2
```

## What’s Binary search tree

Binary search tree has two properties:

- The value of right child is greater than node’s value
- The value of left child is less than node’s value

## Approach #1: Recursive Algorithm

If we traverse each node of binary search tree and comparing the values of `p`

, `q`

and current node’s value, there are four scenarios:

- current node is NULL, we will return NULL as result
- root’s value is in the range of values of
`p`

and`q`

, root is the lowest common ancestor - root’s value is less than
`p`

and`q`

, then lowest common ancestor will be in the left subtree - root’s value is greater than
`p`

and`q`

, then the lowest common ancestor will be the right subtree.

Time Complexity: O(N)

Space Complexity: O(N) for recursion stack.

```
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if(root == NULL || p == NULL || q == NULL)
return NULL; // scenario #1
if((p->val <= root->val && root->val <= q->val) ||
(p->val >= root->val && root->val >= q->val))
return root; // scenario #2
if(p->val < root->val && q->val < root->val) {
return lowestCommonAncestor(root->left, p, q); // scenario #3
}
if(p->val > root->val && q->val > root->val) {
return lowestCommonAncestor(root->right, p, q); // scenario #4
}
return NULL;
}
};
```

## Approach #2: Iterative Algorithm

Iterative algorithm works similarly to the recursive algorithm, but we don’t need a `stack`

to backtrace search. In fact, we only need to find the first node, which `p`

and `q`

begin to split, one of them is in this node’s left subtree, and the other is in the right subtree.

This algorithm is somehow similar to `binary search`

.

Time Complexity: O(N)

Space Complexity: O(1)

```
class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
TreeNode* node = root;
while(node != NULL) {
if(node->val > p->val && node->val > q->val)
node = node->left;
else if(node->val < p->val && node->val < q->val)
node = node->right;
else
return node;
}
return NULL;
}
};
```

When the tree is not a binary search tree, but only a binary tree. The solution would be more complex. Please refer to Lowest Common Ancestor Of A Binary Tree.

The post Lowest Common Ancestor of a Binary Search Tree appeared first on Coder's Cat.

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