题目

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.

分析

合并两个二叉树，根据题目描述，树中对应节点不为空则节点值为二者之和；

递归解决。

代码

/**
 * 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) {
            return t2;
        }else if(t2 == null) {
            return t1;
        }else {
            TreeNode ret = new TreeNode(t1.val + t2.val);
            ret.left = mergeTrees(t1.left,t2.left);
            ret.right = mergeTrees(t1.right,t2.right);
            
            return ret;
        }
    }
}