# 37. 翻转二叉树

java

```java
public class TreeNode {
    public int val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode(int x) {
        val = x;
    }

}


public class TreeUtils {

    public static int k=0;

    public static int k2=0;

    public static TreeNode stringToTree(String s){
        k=0;
        TreeNode root=null;
        root=createTree(root,s);
        return root;
    }

    private static TreeNode createTree(TreeNode node,String s){
        char ch=s.charAt(k++);
        if(ch=='#') return null;
        else{
            node=new TreeNode(ch-'0');
            node.left=createTree(node.left,s);
            node.right=createTree(node.right,s);
            return node;
        }
    }

    public static TreeNode stringToTreeWith(String str,String separator){
        k2=0;
        TreeNode root=null;
        String[] arr=str.trim().split(separator);
        root=createTreeWith(root,arr);
        return root;
    }

    private static TreeNode createTreeWith(TreeNode node,String[] arr){
        String v=arr[k2++];
        if(v.equals("#")) return null;
        else{
            node=new TreeNode(Integer.valueOf(v));
            node.left=createTreeWith(node.left,arr);
            node.right=createTreeWith(node.right,arr);
            return node;
        }
    }

    /**
     * 先序遍历
     * @param root
     */
    public static void travser(TreeNode root){
        if(root!=null){
            System.out.print(root.val+" ");
            travser(root.left);
            travser(root.right);
        }
    }

    /**
     * 中序遍历
     * @param root
     */
    public static void travser2(TreeNode root){
        if(root!=null){
            travser2(root.left);
            System.out.print(root.val+" ");
            travser2(root.right);
        }
    }

    /**
     * 后序遍历
     * @param root
     */
    public static void travser3(TreeNode root){
        if(root!=null){
            travser3(root.left);
            travser3(root.right);
            System.out.print(root.val+" ");
        }
    }
}



/**
 * 226. 翻转二叉树
 *
 * 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
 * 备注:
 * 这个问题是受到 Max Howell 的 原问题 启发的 ：
 *
 * 谷歌：我们90％的工程师使用您编写的软件(Homebrew)，但是您却无法在面试时在白板上写出翻转二叉树这道题，这太糟糕了。
 */
public class Solution226 {
    public TreeNode invertTree(TreeNode root) {
        doThing(root);
        return root;
    }

    public void doThing(TreeNode root) {
        if(root!=null){
            doThing(root.left);
            doThing(root.right);
            TreeNode tmp=root.left;
            root.left=root.right;
            root.right=tmp;
        }
    }

    public static void main(String[] args){
        TreeNode root=TreeUtils.stringToTree("421##3##76##9##");
        TreeNode r=new Solution226().invertTree(root);
        TreeUtils.travser(r);
    }
}
```


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