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Method 1: Recursive post-order traversal of the binary tree: time O(n), space O(n)
/**
* 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)
{
// 1.后序遍历二叉树，交换二叉树的左右指针域的指向
if (root == nullptr)
return root;
mirrorTree(root->left);
mirrorTree(root->right);
swap(root->left, root->right);
return root;
}
};
Method 2: Use the stack to traverse first: time O(n), space O(n)
/**
* 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)
{
// 2.借助栈：先序遍历
if (root == nullptr)
return root;
stack<TreeNode*> stc;
stc.push(root);
while (!stc.empty())
{
TreeNode* node = stc.top();
stc.pop();
if (node->left)
stc.push(node->left);
if (node->right)
stc.push(node->right);
swap(node->left, node->right);
}
return root;
}
};
Method 3: Traversal with the help of queue sequence: time O(n), space O(n)
/**
* 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)
{
// 3.借助队列：层序遍历
if (root == nullptr)
return root;
queue<TreeNode*> que;
que.push(root);
while (!que.empty())
{
TreeNode* node = que.front();
que.pop();
if (node->left)
que.push(node->left);
if (node->right)
que.push(node->right);
swap(node->left, node->right);
}
return root;
}
};