首发于 语雀文档@blueju

    代码：

    1. /**
    2.  * Definition for a binary tree node.
    3.  * class TreeNode {
    4.  *     val: number
    5.  *     left: TreeNode | null
    6.  *     right: TreeNode | null
    7.  *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
    8.  *         this.val = (val===undefined ? 0 : val)
    9.  *         this.left = (left===undefined ? null : left)
    10.  *         this.right = (right===undefined ? null : right)
    11.  *     }
    12.  * }
    13.  */
    15. function mergeTrees(root1: TreeNode | null, root2: TreeNode | null): TreeNode | null {
    16.     if (root1 === null)
    17.         return root2
    18.     if (root2 === null)
    19.         return root1
    20.     root1.val = root1.val + root2.val
    21.     root1.left = mergeTrees(root1.left, root2.left)
    22.     root1.right = mergeTrees(root1.right, root2.right)
    23.     return root1
    24. };