226. Invert Binary Tree [LeetCode]

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Invert a binary tree.反转二叉树

Example:

Input:

     4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

     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.

class Solution {

public:

    TreeNode * invertTree(TreeNode * root) {

         if (root == nullptr) return nullptr;

         invertTree(root-> left);

         invertTree(root-> right);

         swap(root-> left, root-> right);

         return root;

    }

};