226. 翻转二叉树

左子树给右子树，右子树给左子树

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr || (root->left == nullptr && root->right == nullptr))
            return root;
        TreeNode* left = invertTree(root->left);
        TreeNode* right = invertTree(root->right);
        root->left = right;
        root->right = left;
        return root;
    }
};

101. 对称二叉树

挺经典的，有点没写出来，看了下题解

class Solution {
public:
    // 左左和右右 左右和右左
    bool check(TreeNode* left, TreeNode* right)
    {
        if (left == nullptr && right == nullptr) return true;
        if (left == nullptr || right == nullptr) return false;
        if (left->val != right->val) return false;
        bool b1 = check(left->left, right->right);
        bool b2 = check(left->right, right->left);
        return b1 && b2;
    }

    bool isSymmetric(TreeNode* root) {
        return check(root->left, root->right);
    }
};