Invert Binary Tree (E)

Invert a binary tree.

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 f* off.

题意

将二叉树的所有结点左右互换。

思路

简单的递归或者迭代。

代码实现 - 递归

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
}

代码实现 - 迭代

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        Queue<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while (!q.isEmpty()) {
            TreeNode x = q.poll();
            TreeNode temp = x.left;
            x.left = x.right;
            x.right = temp;
            if (x.left != null) {
                q.offer(x.left);
            }
            if (x.right != null) {
                q.offer(x.right);
            }
        }
        return root;
    }
}