leetcode226

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

            TreeNode left = root.left;
            root.left = InvertTree(root.right);
            root.right = InvertTree(left);
            return root;
    }
}

https://leetcode.com/problems/invert-binary-tree/#/description

补充一个使用层次遍历处理的方案，java实现

class Solution {
    public TreeNode invertTree(TreeNode root) {
        List<TreeNode> nodes = new LinkedList<>();
        if (root != null) {
            nodes.add(root);
        }
        while (!nodes.isEmpty()) {
            int numNodes = nodes.size();
            while (numNodes > 0) {
                TreeNode node = nodes.remove(0);
                // push children to the next layer by appending to the end of the list
                if (node.left != null) {
                    nodes.add(node.left);
                }
                if (node.right != null) {
                    nodes.add(node.right);
                }
                // flip
                TreeNode left = node.left;
                node.left = node.right;
                node.right = left;
                numNodes--;
            }
        }
        return root;
    }
}

补充一个python的实现：

class Solution:
    def invertTree(self, root: TreeNode) -> TreeNode:
        if root == None:
            return None
        left = root.left
        root.left = self.invertTree(root.right)
        root.right = self.invertTree(left)
        return root