617. Merge Two Binary Trees leetcode

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.
【617. Merge Two Binary Trees】Example 1:

Input: Tree 1Tree 2 12 / \/ \ 3213 /\\ 547 Output: Merged tree: 3 / \ 45 / \\ 547

Note: The merging process must start from the root nodes of both trees.
三个basecase：
（1） t1 == null && t2 == null
（2） t1 == null && t2 != null
（3） t1 != null && t2 == null
general case :
将右树的值加到左树上，然后，左树的left继续，左树的right继续。。。

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