leetcode 235. 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.

还是bst树，bst树的左节点小于根节点小于右节点，依据此性质递归比较既可以找到公共父节点。注意输入的p,q有可能q->val > p->val或相反

/**
 * 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 == nullptr || p == nullptr || q == nullptr) return nullptr;
        if (max(q->val, p->val) < root->val) {
            return lowestCommonAncestor(root->left, p, q);
        } else if (min(p->val, q->val) > root->val) {
            return lowestCommonAncestor(root->right, p, q);
        } else {
            return root;
        }
    }
};