https://leetcode.com/problems/merge-two-binary-trees/description/

題目：合併2棵二叉樹

思路：採用後序遍歷的方法，分為3種情況：

1 如果t1非空並且t2非空，則t1加上t2的值，返回t1，否則直接返回t1
2 t1為空且t2非空，返回t2
3 t1和t2都為空，返回NULL

程式碼：

class Solution {
public:
    TreeNode *dfs(TreeNode* t1,TreeNode* t2){
              if(!t1&&!t2) return NULL
 
;

              TreeNode *left  = dfs((t1?t1->left:NULL),(t2?t2->left:NULL));
              TreeNode *right = dfs((t1?t1->right:NULL),(t2?t2->right:NULL));

              if(t1){
                 t1->val+=(t2?t2->val:0);
                 t1->left = left;
                 t1->
 
right = right;
              }

             else
                 return t2;

             return t1;
    }
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
              t1 = dfs(t1,t2);
              return t1;
    }
};