[leetCode]Flatten Binary Tree to Linked List

递归的思想，左子树整理成一条链，右子树整理成一条链，然后左子树的尾节点链上右子树的头节点就好了

#include <iostream>
using namespace std;
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;
        flattenTree(root);
    }
    TreeNode *flattenTree(TreeNode *root){
        if(root == NULL) return NULL;
        if(root->left == NULL && root->right == NULL) return root;
        TreeNode *lefthead,*righthead;
        righthead = lefthead = NULL;
        if(root->left != NULL) lefthead = flattenTree(root->left);
        if(root->right != NULL) righthead = flattenTree(root->right);
        if(lefthead == NULL) root->right = righthead;
        if(righthead == NULL) root->right = lefthead;
        if(lefthead != NULL && righthead != NULL){
            root->right = lefthead;
            TreeNode *leftTail = getTail(lefthead);
            leftTail->right = righthead;
        }
        root->left = NULL;
        return root;
    }
    TreeNode *getTail(TreeNode *root){
        if(root == NULL) return NULL;
        while(root->right != NULL) root = root->right;
        return root;
    }
};