leetcode[235]:Lowest Common Ancestor of a Binary Search Tree

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.

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     struct TreeNode *left;
*     struct TreeNode *right;
* };
*/
struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {

if((p->val-root->val)*(q->val-root->val)<=0) return root;
else if(p->val-root->val<0) return lowestCommonAncestor(root->left, p, q);
else return lowestCommonAncestor(root->right, p, q);
}

BST二叉搜索树，左小右大，两个树叶分别在根节点两边返回根节点即可，否则递归寻找下一层。