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.

题目给定一颗二叉搜索树，以及树中两个节点，要求找出两个节点的最近公共祖先，并且定义一个节点是自身的祖先。我们知道二叉树搜索树中，左子树的所有节点小于父节点，右子树的所有节点大于父节点。所以最近公共祖先一定>=min(v,w),并且一定<=max(v,w).

/**
 * 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)
           return root;
        
        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;
        
        
    }
};