学步园

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

其实就是前序遍历，在加上个队尾指针。

/**
 * 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* Tail;
    void preOrder(TreeNode* root)
    {
        if(root != NULL)
        {
            TreeNode* left = root->left;
            TreeNode* right = root->right;
            if(Tail == NULL){
                Tail = root;
            }
            else{
                if(Tail->right != root)
                    Tail->right = root;
                Tail->left = NULL;
                Tail = root;
            }
            preOrder(left);
            preOrder(right);
        }
    }
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        Tail = NULL;
        preOrder(root);
    }
};