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114 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

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

        root->left = NULL;
        if(left)
        {
            root->right = left;
            root = flattenUtil(left); //root指针指向left，即next
        }
        if(right)
        {
            root->right = right;
            root = flattenUtil(right); //root指针指向right，即next
        }
        return root;
    }

    void flatten(TreeNode *root) {
        flattenUtil(root);
    }
};