树的深度

class Solution {
public:
    int maxDepth(TreeNode* root) {
        //空结点没有深度 fast-template
        if(root == NULL)
            return 0;
        //返回子树深度+1
        return max(maxDepth(root->left), maxDepth(root->right)) + 1;
    }
};

合并二叉树

class Solution {
public:
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
        //若只有一个节点返回另一个，两个都为NULL自然返回NULL fast-template
        if (t1 == NULL)
            return t2;
        if (t2 == NULL)
            return t1;
        //根左右的方式递归
        TreeNode* head = new TreeNode(t1->val + t2->val);
        head->left = mergeTrees(t1->left, t2->left);
        head->right = mergeTrees(t1->right, t2->right);
        return head;
    }
};