leetcode刷题之 树(4)-递归:翻转树

[LeetCode] Invert Binary Tree 翻转二叉树

 

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 fuck off.

两种递归的解法：

第一种 递归函数返回翻转完后的根节点 

class Solution {
    public TreeNode invertTree(TreeNode root) {        
        if(root == null)
            return null;
        TreeNode left = root.left;// 后面的操作会改变 left 指针，因此先保存下来
        root.left = invertTree(root.right);
        root.right = invertTree(left);
        return root;        
    }
}

 第二种： 递归函数没有返回值 前序遍历调换左右节点 这个容易理解

class Solution {
    public TreeNode invertTree(TreeNode root) {
        invert(root);
        return root;
        
        
    }
    private void invert(TreeNode root){
        if(root == null)
            return;
        TreeNode node = root.left;
        root.left = root.right;
        root.right = node;
        invert(root.left);
        invert(root.right);
    }
}