27.二叉树的镜像*

class Solution {
public:
    void mirror(TreeNode *root) {
        if(root == nullptr) {
            return;
        }else {
            TreeNode *cur = root->left;
            root->left = root->right;
            root->right = cur;
            mirror(root->left);
            mirror(root->right);
        }
    }
    TreeNode* mirrorTree(TreeNode* root) {
        mirror(root);
        return root;
    }
};

class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        stack<TreeNode*> helpStack;
        TreeNode *cur = root, *tmp = root;
        while(cur != nullptr || !helpStack.empty()) {
            while(cur != nullptr) {
                tmp = cur->left;
                cur->left = cur->right;
                cur->right = tmp;
                helpStack.push(cur);
                cur = cur->left;
            }
            cur = helpStack.top();
            helpStack.pop();
            cur = cur->right;
        }
        return root;
    }
};