class Solution {public:    //目的是将binary tree变成前序遍历的list，但是变换过程是后序遍历    void flatten(TreeNode *root, TreeNode *&first, TreeNode *&last){        if (!root)            return ;        TreeNode *leftFirst = NULL;        TreeNode *leftLast = NULL;        TreeNode *rightFirst = NULL;        TreeNode *rightLast = NULL;        flatten(root->left, leftFirst, leftLast);        //对于向下调用问题，递归的先处理子树        flatten(root->right, rightFirst, rightLast);    //对于向下调用问题，递归的先处理子树        first = root;                                    //对于向上返回问题，first肯定是root        if (root->right)                                //如果有右子树            last = rightLast;        else if (root->left)                            //没有右子树但是有左子树            last = leftLast;        else                                            //没有子树            last = root;        root->left = NULL;        if (leftFirst && !rightFirst){            root->right = leftFirst;        }        else if (!leftFirst && rightFirst){            root->right = rightFirst;        }        else if (leftFirst && rightFirst){            root->right = leftFirst;            leftLast->right = rightFirst;        }        else{            root->right = NULL;        }    }    void flatten(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if (!root)            return ;        TreeNode *first;        TreeNode *last;        flatten(root, first, last);    }};

 

 

 

 

 

 

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