Leetcode 114： Flatten Binary Tree to Linked List

问题描述

思路

java实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        helper(root);
    }
    private TreeNode helper(TreeNode root){
        if (root==null)
            return null;
        //分
        TreeNode leftLast=helper(root.left);
        TreeNode rightLast=helper(root.right);
        //和
        if(leftLast!=null){
            leftLast.right=root.right;//根的右结点连在左结点下
            root.right=root.left;//根的左结点连在右结点下
            root.left=null;//左边置空
        }
        if(rightLast!=null)
            return rightLast;//如果rightLast存在返回rightLast
        if(leftLast!=null)
            return leftLast;//如果leftLast存在返回leftLast
        return root;//leftLast和rightLast都为空
    }
}