题解 | #牛群的树形结构展开#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return TreeNode类
     */
    public TreeNode flattenTree (TreeNode root) {
        // write code here
        if(root == null){
            return null;
        }
        TreeNode result = new TreeNode(-1);
        TreeNode temp = result;
        preOrder(root,temp);
        return result.right;
    }

    public TreeNode preOrder(TreeNode source,TreeNode target){
        if(source == null){
            return target;
        }

        target.right = new TreeNode(source.val);
        target = target.right;
        target = preOrder(source.left,target);
        target = preOrder(source.right,target);
        return target;
    }
}