Leetcode617.Merge Two Binary Trees合并二叉树

给定两个二叉树，想象当你将它们中的一个覆盖到另一个上时，两个二叉树的一些节点便会重叠。

你需要将他们合并为一个新的二叉树。合并的规则是如果两个节点重叠，那么将他们的值相加作为节点合并后的新值，否则不为 NULL 的节点将直接作为新二叉树的节点。

struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
	TreeNode * mergeTrees(TreeNode* t1, TreeNode* t2) {
		if (t1 == NULL && t2 == NULL)
			return NULL;
		if (t1 == NULL)
		{
			t2->left = mergeTrees(t1, t2->left);
			t2 ->right = mergeTrees(t1, t2->right);
			return t2;
		}
		else if (t2 == NULL)
		{
			t1 ->left = mergeTrees(t1 ->left, t2);
			t1 ->right = mergeTrees(t1 ->right, t2);
			return t1;
		}
		else
		{
			t1->val = t1->val + t2->val;
			t1 ->left = mergeTrees(t1->left, t2->left);
			t1 ->right = mergeTrees(t1->right, t2->right);
			return t1;
		}
	}
};