InvertBinaryTree.java

package easy;

import util.TreeNode;
/**
 * 
 * ClassName: InvertBinaryTree
 * @author chenyiAlone  
 * Create Time: 2019/02/27 16:39:04
 * Description: No.226
 *
 *
 * ----------- update -----------------
 * 思路:
 *      简化了原来的题解，其实过程完全一样，只是刚刚发现完全可以不依靠额外的方法，在原方法的基础上递归即可
 *
 *
 *
 * Invert a binary tree.
 *
 * Example:
 *
 * Input:
 *
 *      4
 *    /   \
 *   2     7
 *  / \   / \
 * 1   3 6   9
 * Output:
 *
 *      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 f*** off.
 * 
 */
public class InvertBinaryTree {

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

    public TreeNode invertTreeOld(TreeNode root) {
        helper(root);
        return root;
    }
    private void helper(TreeNode root) {
        if (root == null) return;
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        helper(root.left);
        helper(root.right);
    }
}