17.Merge Two Binary Trees（合并两个二叉树）

Level：

  Easy

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.

思路分析：

  先序遍历两个二叉树，在遍历的过程中更新每个节点的值。

代码：

public class TreeNode{
    int val;
    TreeNode left;
    TreeNode right;
    public TreeNode(int x){
        val=x;
    }
}
public  class Solution{
    public TreeNode mergeBinaryTree(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=t1.val+t2.val;
            t1.left=mergeBinaryTree(t1.left,t2.left);
            t1.right=mergeBinaryTree(t1.right,t2.right);
            return t1;
        }
    }
}