LeetCode: Lowest Common Ancestor of a Binary Search Tree二叉搜索树最近公共祖先

不念不忘少年蓝@ 2021-06-24 16:10 336阅读 0赞

**试题：**  
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 = \[6,2,8,0,4,7,9,null,null,3,5\]

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.  
**代码：**  
如果存在公共祖先，那么这个祖先必然大于左边，小于右边。三种情况互相对立，直接返回结果就行。

/**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    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);
            }else if(root.val<p.val && root.val<q.val){
                return lowestCommonAncestor(root.right, p, q);
            }{
                return root;        
            }
        }
    }