LeetCode-Merge Two Binary Trees

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** Description：**
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.

题意：给定两颗二叉树，要求将两颗树合并成一颗二叉树；合并的规则是将两颗树对应节点的值相加（如果这个节点存在的话）；

解法：这道题主要的难点就在于怎么处理空节点；因为两棵树相同位置上节点可能都存在，也可能只有一颗存在；既然要求新的节点是两个节点值的和，那么如果一棵树对应位置没有节点，我们可以构造一个值为0的节点进行求和计算；

Java

/**
 * 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;
        }
        TreeNode root = new TreeNode(0);
        if (t1 != null) root.val += t1.val;
        else t1 = new TreeNode(0);
        if (t2 != null) root.val += t2.val;
        else t2 = new TreeNode(0);
        root.left = mergeTrees(t1.left, t2.left);
        root.right = mergeTrees(t1.right, t2.right);
        return root;
    }
}