LeetCode-235

235. Lowest Common Ancestor of a Binary Search Tree

原题链接

描述

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).”

示例

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.

思路

对于祖先结点T，一定有 T.val >= p.val && T.val <= q.val。如果 p、q 的 val 都小于 root.val，那么p、q 的 LCA 一定在 root 的左子树上；如果 p、q 的 val 都大于 root.val，那么p、q 的 LCA 一定在 root 的右子树上；否则一旦出现其他情况，那么该结点就是 p、q 的 LCA。

解答

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) return null;
        if (root.val >p.val && root.val>q.val) return lowestCommonAncestor(root.left,p,q);
        if (root.val < p.val && root.val < q.val) return lowestCommonAncestor(root.right,p,q);

        return root;
    }
}