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题目

方法一：递归

var mergeTrees = function(root1, root2) {
  if(!root1)return root2;
  if(!root2)return root1;
  let temp = root1.val+root2.val;
  root1.val = temp;
  root1.left = mergeTrees(root1.left,root2.left);
  root1.right = mergeTrees(root1.right,root2.right);
  return root1;
};