617. 合并二叉树

前中后序遍历都可，注意终止条件，当一棵树为NULL时返回另一棵树

  1. class Solution {
  2. public:
  3.     TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
  4.         if(!root1 && !root2)return NULL;
  5.         else if(!root1) 
  6.             return root2;
  7.         else if(!root2)
  8.             return root1;
  9.         else    
  10.             root1->val += root2->val;
  12.         root1->left =  mergeTrees(root1->left,root2->left);
  13.         root1->right = mergeTrees(root1->right,root2->right);
  14.         return root1;
  15.     }
  16. };