617-合并二叉树

LeetCode 617. Merge Two Binary Trees

LeetCode-617

You are given two binary trees root1 and root2.

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 the two trees 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 the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]

method: 递归

递归结束条件：如果一个节点为空，返回另一棵树的节点，如果他也是空，那这个位置就是空节点

TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
    if (root1 == nullptr) return root2;
    if (root2 == nullptr) return root1;
    root1->val += root2->val;   // 直接在root1上改
    root1->left = mergeTrees(root1->left, root2->left);
    root1->right = mergeTrees(root1->right, root2->right);
    return root1;
}