二叉搜索树的最近公共祖先 Lowest Common Ancestor of a Binary Search Tree

lowest-common-ancestor-of-a-binary-search-tree

题目

给定一个二叉搜索树, 找到该树中两个指定节点的最近公共祖先。

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] binarysearchtree_improved.png

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.

https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

思路&题解

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

知识点分析

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