力扣226. 翻转二叉树

递归

• 思路：
  • 从根开始递归遍历二叉树，叶节点开始翻转；
  • 如果遍历到的当前的 root 节点左右两棵子树已经翻转，交换左右子树即可；

/**
 * 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:
    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr) {
            return nullptr;
        }

        TreeNode* left = invertTree(root->left);
        TreeNode* right = invertTree(root->right);
        root->left = right;
        root->right = left;

        return root;
    }
};