Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked ListOct 14 '12

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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    void flattenInt(TreeNode *root)
    {       
        if (root->left) {
            flattenInt(root->left); 
        
            TreeNode *temp=root->right;
            TreeNode *trav=root->left;

            /* Attach the right child of this node to
             * the rightmost node of the left child 
             */
            while (trav->right)
                trav = trav->right;        

            root->right=root->left;
            trav->right=temp;
            root->left=NULL;
        }  
        if (root->right)
            flattenInt(root->right);
    }
public:
    void flatten(TreeNode *root) {
        if (!root)
            return;
        if (!root->left && !root->right)
            return;
        flattenInt(root);
        
    }
};

/* Alternative Solution */

class Solution {
    void flattenInt(TreeNode *root, TreeNode **tail) {
        if (root) {
            TreeNode *right = root->right;
            TreeNode *left  = root->left;
            if (*tail) {
                (*tail)->right = root;
                (*tail)->left  = nullptr;
            }
            *tail = root;
            flattenInt(left, tail);
            flattenInt(right, tail);
        }
    }
public:
    void flatten(TreeNode *root) {
        TreeNode *tail=nullptr;
        flattenInt(root, &tail);
    }
};