617. Merge Two Binary Trees

• Given two binary trees and imagine that when you put one of them to
  cover the other, some nodes of the two trees are overlapped while the
  others are not.

  You need to merge them into a new binary tree. The merge rule is that
  if two nodes overlap, then sum node values up as the new value of the
  merged node. Otherwise, the NOT null node will be used as the node of
  new tree.

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     struct TreeNode *left; *     struct TreeNode *right; * }; */struct TreeNode* mergeTrees(struct TreeNode* t1, struct TreeNode* t2) {    struct TreeNode * newNode = NULL;    if(t1 == NULL && t2 == NULL){        return NULL;    }    if(t1 == NULL){        return mergeTrees(t2,t1);    }    if(t2 == NULL){        return t1;    }    t1->val += t2->val;    t1->left = mergeTrees(t1->left,t2->left);    t1->right = mergeTrees(t1->right,t2->right);    return t1;}