题目 dfs一下

cpp
class Solution {
public:
    TreeNode* reverseOddLevels(TreeNode* root) {
        dfs(root->left, root->right, true);
        return root;
    }

    void dfs(TreeNode *root1, TreeNode *root2, bool isOdd) {
        if (root1 == nullptr) {
            return;
        }
        if (isOdd) {
            swap(root1->val, root2->val);
        }
        dfs(root1->left, root2->right, !isOdd);
        dfs(root1->right, root2->left, !isOdd);
    }
};

层序遍历一下

cpp
class Solution {
public:
    TreeNode* reverseOddLevels(TreeNode* root) {
        queue<TreeNode *> qu;
        qu.emplace(root);
        bool isOdd = false;
        while (!qu.empty()) {
            int sz = qu.size();
            vector<TreeNode *> arr;
            for (int i = 0; i < sz; i++) {
                TreeNode *node = qu.front();
                qu.pop();
                if (isOdd) {
                    arr.emplace_back(node);
                }
                if (node->right||node -> left) {
                    qu.emplace(node->left);
                    qu.emplace(node->right);
                }
            }
            if (isOdd) {
                for (int l = 0, r = sz - 1; l < r; l++, r--) {
                    swap(arr[l]->val, arr[r]->val);
                }
            }            
            isOdd ^= true;
        }
        return root;
    }
};