lowestCommonAncestor.c

// https://leetcode.com/problems/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;

 * };

 */

#include <stdio.h>
 struct TreeNode {

      int val;

      struct TreeNode *left;

      struct TreeNode *right;

  };

struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode*
        p, struct TreeNode* q) {
    if(root == NULL || p==NULL || q==NULL)
        return NULL;
    if((p->val < root->val) && (q->val < root->val)) // 都在left subtree
        return lowestCommonAncestor(root->left, p, q);
    if((p->val > root->val) && (q->val > root->val)) // 都在left subtree
        return lowestCommonAncestor(root->right, p, q);
    if((p->val - root->val) * (q->val - root->val)<=0) // 一个在左，一个在右
        return root;
}

//
// note: 正因为是BST，所以可以这么做，如果是普通的二叉树，就是个挺难的题目了。