Flatten Binary Tree to Linked List

flatten_binary_tree_to_linked_list.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}\ 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

 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        
        //root -> leftSon -> ... -> lastLeft -> right -> ... -> lastRight
        
        if (!root)
            return;
        TreeNode *last=NULL;
        root = flatten(root, last);
        
        return ;
    }
    TreeNode *flatten(TreeNode *root, TreeNode *&last) {
        TreeNode *left = root->left;    root->left = NULL;
        TreeNode *right = root->right;  root->right = NULL;
        TreeNode *lastLeft=NULL;
        if (left) {
            root->right = flatten (left, lastLeft);
        }
        TreeNode  *lastRight=NULL;
        if (right)  {
            if (lastLeft)
                lastLeft->right = flatten (right, lastRight);
            else
                root->right = flatten(right, lastRight);
        }
        
        //update last
        if (lastRight)
            last = lastRight;
        else if (lastLeft)
            last = lastLeft;
        else
            last = root;
        return root;
    }

};