LeetCode算法题114：二叉树展开为链表解析

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
将其展开为：

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

这个题的思路是直接把左子树插入到根节点与右子树之间，可以用迭代来做。

C++源代码：

/**
 * 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:
    void flatten(TreeNode* root) {
        TreeNode* cur = root;
        while(cur){
            if(cur->left){
                TreeNode* tmp = cur->left;
                while(tmp->right) tmp = tmp->right;
                tmp->right = cur->right;
                cur->right = cur->left;
                cur->left = NULL;
            }
            cur = cur->right;
        }
    }
};

python3源代码：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def flatten(self, root: TreeNode) -> None:
        """
        Do not return anything, modify root in-place instead.
        """
        cur = root
        while cur:
            if cur.left:
                tmp = cur.left
                while tmp.right:
                    tmp = tmp.right
                tmp.right = cur.right
                cur.right = cur.left
                cur.left = None
            cur = cur.right