LeetCode - Convert Binary Search Tree to Sorted Doubly Linked List

解法一：inorder recursion

class Solution {
public:
    Node* treeToDoublyList(Node* root) {
        if(!root) return nullptr;
        Node* head=NULL, *pre=NULL;
        inorder(root, pre, head); 
        pre->right = head;
        head->left = pre;
        return head;
    }
    void inorder(Node* root, Node*& pre, Node*& head){
        if(!root) return;
        inorder(root->left, pre, head);
        if(!head) {
            head = root;
            pre = root;
        }
        else{
            pre->right = root;
            root->left = pre;
            pre = root;
        }
        inorder(root->right, pre, head);
    }
};

解法二：inorder non-recrusion stack

class Solution {
public:
    Node* treeToDoublyList(Node* root) {
        if(!root) return nullptr;
        stack<Node*> st;
        Node* head = NULL, *pre=NULL;
        while(!st.empty() || root){
            while(root){               //left subtreee
                st.push(root);
                root=root->left;
            }
            root = st.top(); st.pop(); //current node
            if(!head){
                head = root;
                pre = root;
            }else{
                pre->right = root;
                root->left = pre;
                pre = root;
            }
            root=root->right;             //right
        }
        pre->right = head;
        head->left = pre;
        return head;
    }

};

解法三：divide and conquer, recursion