Leetcode 226 翻转二叉树

Leetcode 226翻转二叉树

数据结构定义：

翻转一棵二叉树。

示例：

输入：

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

     4
   /   \
  7     2
 / \   / \
9   6 3   1
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

自底向上递归：

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

自顶向下递归：

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

迭代：

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