Leetcode 235. Lowest Common Ancestor of a Binary Search Tree

lca算法思路总结：

思想：二叉查找树的性质，如果是祖先，和左右子树的差值乘积为负数。

如果在左边，往左边进行递归。如果往右边，往右边递归。

/**
 * 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* ans = NULL;
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        lca(root, p , q);
        return ans;
    }
    void lca(TreeNode* root, TreeNode* p, TreeNode* q) {
        if((root->val - p->val) * (root->val - q->val) <= 0) ans = root;
        else if(root->val < p->val && root->val < q->val) lca(root->right, p, q);
        else lca(root->left, p ,q);
    }
};