LeetCode:Lowest Common Ancestor of a Binary Tree二叉树最近公共祖先

雨点打透心脏的1/2处 2021-06-24 16:10 308阅读 0赞

**试题：**  
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.  
**代码：**  
最低公共祖先有个特点，就是左树和右树同时包含两个节点。更高祖先都没有这一特点。另外一种情况就是p是q的父节点或q是p的父节点。

/**
     * 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 || root==p || root==q){
                return root;
            }
            TreeNode left = lowestCommonAncestor(root.left, p, q);
            TreeNode right = lowestCommonAncestor(root.right, p, q);
            if(left!=null && right!=null){
                return root;
            }else if(left==null && right!=null){
                return right;
            }else if(left!=null && right==null){
                return left;
            }else{
                return null;
            }
        }
    }