226-Invert Binary Tree

Description

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 f*** off.

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問題描述

反轉二叉樹。

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問題分析

在反轉的過程中， 注意先儲存左分支。

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解法1

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;

        Queue<TreeNode> queue = new LinkedList();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode current = queue.poll();
            TreeNode temp = current.left;
            current.left = current.right;
            current.right = temp;
            if (current.left != null) queue.add(current.left);
            if (current.right != null) queue.add(current.right);
        }

        return root;
    }
}

解法2

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return null;

        TreeNode tmp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(tmp);

        return root;
    }
}