# int32bit / leetcode

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tree.cpp

## Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

``````        _______6______
/              \
___2__          ___8__
/      \        /      \
0      _4       7       9
/  \
3   5
``````

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

## Solution

```class Solution {
public:
TreeNode *lowestCommonAncestor(TreeNode *root, TreeNode *p, TreeNode *q) {
if (root == nullptr)
return nullptr;
return lowestCommonAncestor(root, p->val, q->val);
}
private:
TreeNode *lowestCommonAncestor(TreeNode *root, int v1, int v2) {
if (root->val > v1 && root->val > v2)
return lowestCommonAncestor(root->left, v1, v2);
if (root->val < v1 && root->val < v2)
return lowestCommonAncestor(root->right, v1, v2);
return root;
}
};```

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