剑指offer_18:二叉树的镜像

请完成一个函数，输入一个二叉树，该函数输出它的镜像。

例如输入：

镜像输出：

示例 1：
输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

限制：
0 <= 节点个数 <= 1000

1、递归

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

2、DFS

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if(root==null) return null;
        Stack<TreeNode> stack=new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node=stack.pop();
            if(node.left!=null) stack.push(node.left);
            if(node.right!=null) stack.push(node.right);
            TreeNode temp=node.left;
            node.left=node.right;
            node.right=temp;
        }
        return root;
    }
}

3、BFS

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        if(root==null) return null;
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode node=queue.poll();
            TreeNode t=node.left;
            node.left=node.right;
            node.right=t;
            if(node.left!=null) queue.offer(node.left);
            if(node.right!=null) queue.offer(node.right);
        }
        return root;
    }
}