merge-two-binary-trees.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * 合并二叉树
 * @param {TreeNode} root1
 * @param {TreeNode} root2
 * @return {TreeNode}
 */
var mergeTrees = function (root1, root2) {
  // 精简一下判断逻辑
  if (!root1) return root2;
  if (!root2) return root1;

  // 合并节点值到 root1
  root1.val += root2.val;
  // 递归合并左右子树
  root1.left = mergeTrees(root1.left, root2.left);
  root1.right = mergeTrees(root1.right, root2.right);

  return root1;
};