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.

解题思路;
（1）如果当前结点的值比两个结点的值都大，则最小公共祖先肯定在左子树上
（2）如果当前结点的值比两个结点的值都小，则最小公共祖先肯定在右子树上
（3）否则，最小公共祖先就是该结点

struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {

if (root == NULL || p == NULL || q == NULL)    return NULL;struct TreeNode* commonAncestor = NULL;if ((root->val > p->val) && (root->val > q->val)){    commonAncestor = lowestCommonAncestor(root->left, p, q);}else if ((root->val < p->val) && (root->val < q->val)){    commonAncestor = lowestCommonAncestor(root->right, p, q);}else    commonAncestor = root;return commonAncestor;

}