1. 递归

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    TreeNode* Mirror(TreeNode* pRoot) {
        if (pRoot == nullptr) return pRoot;
        swap(pRoot->left, pRoot->right);
        Mirror(pRoot->left);
        Mirror(pRoot->right);
        return pRoot;
    }
};

2. 层序遍历

class Solution {
public:
    TreeNode* Mirror(TreeNode* pRoot) {
        if (pRoot == nullptr) return pRoot;
        queue<TreeNode*> q;
        q.push(pRoot);
        while (!q.empty()) {
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                TreeNode* tmp_node = q.front();
                q.pop();
                swap(tmp_node->left, tmp_node->right);
                if (tmp_node->left) q.push(tmp_node->left);
                if (tmp_node->right) q.push(tmp_node->right);
            }
        }
        return pRoot;
    }
};