[leetcode]Flatten Binary Tree to Linked List

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);

    }
};

EOF