Flatten Binary Tree to Linked List

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

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

思路：给定一个二叉树，将它变成一个类似上面那样的链表，而且操作要原地进行.递归处理之后，把右子树连接到处理后的左子树上，然后在把左子树改成右子树，然后再把左子树设为NULL；

/**
 * 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 flattree(TreeNode *root)
    {
        if(root==NULL)
            return;
        flattree(root->left);
        flattree(root->right);
        TreeNode *cur=root;
        if(cur->left==NULL)
            return;
        else
            cur=cur->left;
        while(cur->right!=NULL)
            cur=cur->right;
        cur->right=root->right;
        root->right=root->left;
        root->left=NULL;
    }
    void flatten(TreeNode *root) {
        flattree(root);
    }
};

循环方法：

/**
 * 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) {
        if(root==NULL)
            return ;
        TreeNode *pPre;
        while(root)
        {
            if(root->left) {
                pPre=root->left;
                while(pPre->right)
                    pPre=pPre->right;
                pPre->right=root->right;
                root->right=root->left;
                root->left=NULL;
            }
            root=root->right;
        }
    }
};