617. 合并二叉树

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    public:
    // 二叉树，递归
    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
        if(root1==nullptr){
            return root2;
        }
        if(root2==nullptr){
            return root1;
        }
        // 执行走到这，root1和root2都不为空
        // 前序遍历 （根，左，右）
        root1->val+=root2->val;
        root1->left=mergeTrees(root1->left,root2->left);
        root1->right=mergeTrees(root1->right,root2->right);
        return root1;
    }
};