二叉树展开成链表

发表于

给定一个二叉树，原地将它展开为链表。
例如，给定二叉树
1
/ \
2 5
/ \ \
3 4 6
展开为：
1-2-3-4-5-6

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* last;
    void flatten(TreeNode* root) {
        if (root == NULL) return;
        flatten(root->right);
        flatten(root->left);
        root->right = last;
        root->left = NULL;
        last = root;
    }
};

链接：https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list