235. Lowest Common Ancestor of a Binary Search Tree

[fockspaces]

Given a binary search tree (BST), find the lowest common ancestor (LCA) node 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).”

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.

[fockspaces]

因為樹是BST，共同parent node條件為 p->val <= root->val <= q->val 先將p < q後，開始recursion查找

/**
 * 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(p->val > q->val) return lowestCommonAncestor(root, q, p);
        if(!root) return NULL;
        if(p->val <= root->val && root->val <= q->val) return root;
        if(p->val > root->val) 
            return lowestCommonAncestor(root->right, p, q);
        return lowestCommonAncestor(root->left, p, q);
    }
};