LeetCode 114: Flatten Binary Tree to Linked List

Difficulty: 3

Frequency: 3

Problem:

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

For example,
Given

         1        / \       2   5      / \   \     3   4   6

The flattened tree should look like:

   1    \     2      \       3        \         4          \           5            \             6

Solution:

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void flatten(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        flattenRecursive(root);    }    TreeNode * flattenRecursive(TreeNode * root)    {        if (root==NULL)            return NULL;                TreeNode * left = flattenRecursive(root->left);        TreeNode * right = flattenRecursive(root->right);                TreeNode * traverse = root;        traverse->right = left;        while(traverse->right!=NULL)        {            traverse = traverse->right;        }        traverse->right = right;        root->left = NULL;// This one is important.        return root;    }};

Notes: