LeetCode(617) Merge Two Binary Trees

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本文介绍了一种将两棵二叉树合并成一棵新二叉树的算法。当两棵树的节点重叠时，新树的节点值为两节点值之和；若一树的节点为空，则采用另一树的节点值。通过递归方式实现该算法。

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;
        }
    }
}