leetcode No617. Merge Two Binary Trees

Posted 2022-06-11 Dufre.WC

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.

Algorithm

这一题很自然的想到递归，这是看到二叉树的第一反应，然后就是如何使代码更加简洁

Code

/**
 * Definition for a binary tree node.
 * 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;
        else if (t1==NULL)
            return t2;
        
        else if (t2==NULL)
            return t1;
        
        else
            t1->val += t2->val;
            t1->left = mergeTrees(t1->left, t2->left);
            t1->right = mergeTrees(t1->right, t2->right);
        
        return t1;  
    
;