【LeetCode】617.Merge Two Binary Trees(Easy)解题报告

郝春雨

于 2018-03-17 20:18:43 发布

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分类专栏： LeetCode Tree 文章标签： leetcode tree

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本文针对LeetCode上的合并二叉树问题提供了一种递归解决方案。当两棵树重叠时，将节点值相加作为新节点值；若不重叠，则使用非空节点值。此方法从根节点开始合并，时间复杂度为O(n)，空间复杂度也为O(n)。

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【LeetCode】617.Merge Two Binary Trees(Easy)解题报告

题目地址：https://leetcode.com/problems/merge-two-binary-trees/description/
题目描述：

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.

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 time : O(n)
 space : O(n)
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1 == null) return t2;
        if(t2 == null) return t1;
        TreeNode newNode = new TreeNode(t1.val + t2.val);
        newNode.left = mergeTrees(t1.left,t2.left);
        newNode.right = mergeTrees(t1.right,t2.right);
        return newNode;
    }
}

Date：2018年3月17日