Merge Two Binary Trees

题目描述：

You are given two binary trees root1 and root2.

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 the two trees 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 the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Example 1:

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]
Example 2:

Input: root1 = [1], root2 = [1,2]
Output: [2,2]

思路:

dfs题，递归合成二叉树

代码:

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func mergeTrees(root1 *TreeNode, root2 *TreeNode) *TreeNode {
    var dfs func(root1, root2 *TreeNode)*TreeNode
    dfs = func(root1, root2 *TreeNode)*TreeNode{
        if root1 == nil || root2 == nil{
            if root1 == nil{
                return root2
            }else{
                return root1
            }
        }
        root1.Val += root2.Val
        root1.Left = dfs(root1.Left, root2.Left)
        root1.Right = dfs(root1.Right, root2.Right)
        
        return root1
    }
    return dfs(root1, root2)
}

代码效率：

执行用时：12 ms, 在所有 Go 提交中击败了95.45%的用户
内存消耗：6.7 MB, 在所有 Go 提交中击败了91.63%的用户