[二叉树]617. Merge Two Binary Trees

题目：617. Merge Two Binary Trees

一般的二叉树合并。若两个node重叠，则value等于它们的和。
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.

思路：同时遍历两棵树。
左树为空就返回右树，反之返回左树，两树不空就返回和。

class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1 == null){return t2;}
        if(t2 == null){return t1;}
        
        TreeNode t3 = new TreeNode(0);
        t3.val = t1.val + t2.val;
        t3.left = mergeTrees(t1.left, t2.left);
        t3.right = mergeTrees(t1.right, t2.right);
        
        return t3;

    }
}