二叉树的镜像

力扣地址

递归：

将左右子树递归调换即可实现镜像

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        if (!root) return root;
        TreeNode* node = root->left;
        root->left = mirrorTree(root->right);
        root->right = mirrorTree(node);
        return root;
    }
};

非递归：

用栈实现，将左右子树押栈再弹栈再交换

此处用vector模拟stack

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        if (!root) return root;
        TreeNode* res = root;
        vector<TreeNode*> s{root};
        while (!s.empty()) {
            root = s.back();
            s.pop_back();
            
            TreeNode* node = root->left;
            root->left = root->right;
            root->right = node;

            if (root->left) s.push_back(root->left);
            if (root->right) s.push_back(root->right);
        }
        return res;
    }
};