235. Lowest Common Ancestor of a Binary Search Tree [LeetCode]

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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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Given binary search tree:  root = [6,2,8,0,4,7,9,null,null,3,5]

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

Example 1:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6.

Example 2:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself 
             according to the LCA definition.

Note:

• All of the nodes' values will be unique.

• p and q are different and both values will exist in the BST.

给定一颗二分搜索树和两个节点，寻找这两个节点的最近公共祖先。

class Solution {

public:

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

         assert(p &&q);

         if (root == nullptr) return nullptr;

         if (p-> val < root-> val &&q-> val < root-> val)

             return lowestCommonAncestor(root-> left, p, q);

         if (p-> val > root-> val &&q-> val > root-> val)

             return lowestCommonAncestor(root-> right, p, q);

         return root;

    }

};