【LeetCode】 114. Flatten Binary Tree to Linked List 二叉树展开为链表（Medium）（JAVA）

【LeetCode】 114. Flatten Binary Tree to Linked List 二叉树展开为链表（Medium）（JAVA）

题目地址: https://leetcode.com/problems/flatten-binary-tree-to-linked-list/

题目描述：

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

题目大意

给定一个二叉树，原地将它展开为链表。

解题方法

1、展开链表是一个前序遍历
2、直接用中序遍历展开即可

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    TreeNode pre = null;
    public void flatten(TreeNode root) {
        fH(root);
    }

    public void fH(TreeNode root) {
        if (root == null) return;
        TreeNode left = root.left;
        TreeNode right = root.right;
        if (pre != null) {
            pre.right = root;
            pre.left = null;
        }
        pre = root;
        fH(left);
        fH(right);
    }
}

执行用时 : 0 ms, 在所有 Java 提交中击败了 100.00% 的用户
内存消耗 : 39.7 MB, 在所有 Java 提交中击败了 6.67% 的用户