617. Merge Two Binary Trees（Tree）

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

题目：合并2棵二叉树

思路：采用后序遍历的方法，分为3种情况：

1 如果t1非空并且t2非空，则t1加上t2的值，返回t1，否则直接返回t1
2 t1为空且t2非空，返回t2
3 t1和t2都为空，返回NULL

代码：

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;
    }
};