Algorithem_ Merge Two Binary Trees

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.

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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]

solution

Using recursion , You can see from the above example , The basic calculation is the addition of two values , If I have a theta null, And 0 Add up .

Code logic ：

Declare a new TreeNode,TreeNode Of val = root1.val + root2.val,TreeNode Of left= recursive (root1.left, roo2.left),right= recursive (root1.right, root2.right)

The code is as follows ：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init() { self.val = 0; self.left = nil; self.right = nil; }
 *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
 *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
 *         self.val = val
 *         self.left = left
 *         self.right = right
 *     }
 * }
 */
class Solution {
    func mergeTrees(_ root1: TreeNode?, _ root2: TreeNode?) -> TreeNode? {
        if root1 == nil && root2 == nil {
            return nil
        }
        
        let valValue = (root1 == nil ? 0 : root1!.val) + (root2 == nil ? 0 : root2!.val)
        let newNode = TreeNode(valValue)
        newNode.left = mergeTrees(root1?.left, root2?.left)
        newNode.right = mergeTrees(root1?.right, root2?.right)
        return newNode
    }
}