[LeetCode] Flatten Binary Tree to Linked List

1 迭代，重要关系  

p->right = s.top();

class flat{
    public:
        void flatten(TreeNode* root) {
            if (root == NULL) return;
            stack<TreeNode*> s;
            s.push(root);
            while (!s.empty()) {
                TreeNode *p = s.top();
                s.pop();
                if (p->right)
                    s.push(p->right);
                if (p->left)
                    s.push(p->left);
                p->left = NULL;
                if (!s.empty())
                    p->right = s.top();
            }
        }
};

2 递归 

思路：

1 搞定左边

2 搞定右边

3 将左边的插入到root 和right中间

　　3.1 找到左边的最后一个节点 left_most

　　3.2 left_most->right = root->right;

　　3.3 root->right = left

class flat{
    public:
            void  flatten_rec(TreeNode* root) {
            if (root == NULL) return;
            if(root->left == NULL && root->right == NULL)
                return ;

            flatten_rec(root->left);
            flatten_rec(root->right);
            
            TreeNode * left_most = root->left;
            while(left_most && left_most->right)
            {
                left_most = left_most->right;
            }
            if(root->left != NULL)
            {
                left_most->right = root->right;
                root->right = root->left;
            }

            root->left = NULL;
            return ;

        }
};