[LeetCode] Construct String from Binary Tree

Invert a binary tree.

     4
   /     2     7
 / \   / 1   3 6   9

to

     4
   /     7     2
 / \   / 9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

反转一个二叉树，使用递归很容易就可以实现。递归交换节点即可。

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr)
            return 0;
        root->left = invertTree(root->left);
        root->right = invertTree(root->right);
        swap(root->left, root->right);
        return root;
    }
};
// 0 ms

使用迭代实现

class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == nullptr)
            return 0;
        stack<TreeNode*> s;
        s.push(root);
        while (!s.empty()) {
            TreeNode* node = s.top();
            s.pop();
            if (node->left != nullptr)
                s.push(node->left);
            if (node->right != nullptr)
                s.push(node->right);
            swap(node->left, node->right);
        }
        return root;
    }
};
// 0 ms