LeetCode: Merge Two Binary Trees

Introduction

ツリーの問題です。バイナリツリーをマージする、比較的簡単な問題です。

Problem statement

Given two binary trees and imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not.

You need to merge them into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of new tree.

Example 1:

Input: 
    Tree 1                     Tree 2                  
          1                         2                             
         / \                       / \                            
        3   2                     1   3                        
       /                           \   \                      
      5                             4   7                  
Output: 
Merged tree:
         3
        / \
       4   5
      / \   \ 
     5   4   7


Note: The merging process must start from the root nodes of both trees.

My solution

1. t1ノード, t2ノードともに存在していたら t1.val += t2.val を実行します。t1とt2ともにNULL,t1のみnull, t2のみnullの3ケースに対しても対応します。
2. 左側・右側のノードでも1と同一のことを実行します。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) {
            return null;
        }
        if (t1 == null) {
            return t2;
        }
        if (t2 == null) {
            return t1;
        }
        t1.val += t2.val;
        t1.right = mergeTrees(t1.right, t2.right);
        t1.left = mergeTrees(t1.left, t2.left);
        return t1;
    }
}