LeetCode实战：二叉树的最近公共祖先

背景

题目英文

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

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 the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.

Example 2:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, 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 binary tree.

题目中文

给定一个二叉树, 找到该树中两个指定节点的最近公共祖先。

百度百科中最近公共祖先的定义为：“对于有根树 T 的两个结点 p、q，最近公共祖先表示为一个结点 x，满足 x 是 p、q 的祖先且 x 的深度尽可能大（一个节点也可以是它自己的祖先）。”

例如，给定如下二叉树: root = [3,5,1,6,2,0,8,null,null,7,4]

示例 1:

输入: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
输出: 3
解释: 节点 5 和节点 1 的最近公共祖先是节点 3。

示例 2:

输入: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
输出: 5
解释: 节点 5 和节点 4 的最近公共祖先是节点 5。因为根据定义最近公共祖先节点可以为节点本身。

说明:

所有节点的值都是唯一的。
p、q 为不同节点且均存在于给定的二叉树中。

算法实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
 
public class Solution
{
    public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q)
    {
        return Find(root, p, q);
    }

    private TreeNode Find(TreeNode current, TreeNode p, TreeNode q)
    {
        if (current == null || current == p || current == q)
            return current;
        TreeNode left = Find(current.left, p, q);
        TreeNode right = Find(current.right, p, q);
        if (left != null && right != null)
            return current;
        return left != null ? left : right;
    }
}

实验结果

状态：通过
31 / 31 个通过测试用例
执行用时: 132 ms, 在所有 C# 提交中击败了 96.10% 的用户
内存消耗: 27.5 MB, 在所有 C# 提交中击败了 20.00% 的用户

提交结果