Lowest Common Ancestor of a Binary Search Tree

        _______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;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root == NULL || p == NULL || q == NULL)
        return NULL;
        else if (min(p->val ,q->val)>root->val)
          lowestCommonAncestor( root->right,  p,  q);
        else if (max(p->val ,q->val)<root->val)
          lowestCommonAncestor( root->left,  p,  q);
        else
        return root;
       
    }
};

Lowest Common Ancestor of a Binary Search Tree

原文：http://www.cnblogs.com/gofighting/p/5036107.html