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Leetcode 226 Invert Binary Tree

发表于 更新于 分类于 Valine：
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Invert a binary tree.

Example:

Input:

1
2
3
4
5
     4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

1
2
3
4
5
     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.

1 递归

这太简单了…

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return root;
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}

2 队列

再放一个非递归用队列实现的吧. 其实楼主在刻意的练习二叉树的非递归算法, 怕面试的时候简单题不让用递归, 不会的话就gg了.

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null)
            return root;
        Queue<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while(!q.isEmpty())
        {
            int size = q.size();
            while(size != 0)
            {
                TreeNode tmp = q.poll();
                TreeNode dummy = tmp.left;
                tmp.left = tmp.right;
                tmp.right = dummy;
                if(tmp.left != null)
                    q.offer(tmp.left);
                if(tmp.right != null)
                    q.offer(tmp.right);
                --size;
            }
        }
        return root;
    }
}