LeetCode 1123. Lowest Common Ancestor of Deepest Leaves

Posted 2021-03-23 Dylan_Java_NYC

原题链接在这里：https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/

题目：

Given a rooted binary tree, return the lowest common ancestor of its deepest leaves.

Recall that:

• The node of a binary tree is a leaf if and only if it has no children
• The depth of the root of the tree is 0, and if the depth of a node is d, the depth of each of its children is d+1.
• The lowest common ancestor of a set S of nodes is the node A with the largest depth such that every node in S is in the subtree with root A.

 

Example 1:

Input: root = [1,2,3]
Output: [1,2,3]
Explanation: 
The deepest leaves are the nodes with values 2 and 3.
The lowest common ancestor of these leaves is the node with value 1.
The answer returned is a TreeNode object (not an array) with serialization "[1,2,3]".

Example 2:

Input: root = [1,2,3,4]
Output: [4]

Example 3:

Input: root = [1,2,3,4,5]
Output: [2,4,5]

Constraints:

• The given tree will have between 1 and 1000 nodes.
• Each node of the tree will have a distinct value between 1 and 1000.

题解：

For dfs, state needs current TreeNode root only. return value needs both depth and lca of deepest leaves node.

Divide and conquer, have left result and right result first. Then compare the depth, pick the bigger side, if both sides have same depth, return root with depth + 1.

Time Complexity: O(n). n is nodes count of the whole tree.

Space: O(logn).

AC Java:

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public TreeNode lcaDeepestLeaves(TreeNode root) {
12         if(root == null){
13             return root;
14         }
15         
16         return lcaDfs(root).node;
17     }
18     
19     private Pair lcaDfs(TreeNode root){
20         if(root == null){
21             return new Pair(root, 0);
22         }
23         
24         Pair l = lcaDfs(root.left);
25         Pair r = lcaDfs(root.right);
26         if(l.depth > r.depth){
27             return new Pair(l.node, l.depth + 1);
28         }else if(l.depth < r.depth){
29             return new Pair(r.node, r.depth + 1);
30         }else{
31             return new Pair(root, l.depth + 1);
32         }
33     }
34 }
35 
36 class Pair{
37     TreeNode node;
38     int depth;
39     public Pair(TreeNode node, int depth){
40         this.node = node;
41         this.depth = depth;
42     }
43 }