直接递归即可
TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
TreeNode *t3;
TreeNode *l1,*r1;
TreeNode *l2,*r2;
if(t1!=NULL&&t2!=NULL){
t3=new TreeNode(t1->val+t2->val);
l1=t1->left;
l2=t2->left;
r1=t1->right;
r2=t2->right;
}
if(t1==NULL&&t2==NULL)
return NULL;
if(t1==NULL&&t2!=NULL){
t3=new TreeNode(t2->val);
l1=NULL;
l2=t2->left;
r1=NULL;
r2=t2->right;
}
if(t1!=NULL&&t2==NULL){
t3=new TreeNode(t1->val);
l1=t1->left;
l2=NULL;
r1=t1->right;
r2=NULL;
}
t3->left=mergeTrees(l1,l2);
t3->right=mergeTrees(r1,r2);
return t3;
}