二叉树由有序链表和有序数组转换

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        TreeNode *root=NULL;
        if(head==NULL){
            return root;
        }else if(head->next==NULL){
            return new TreeNode(head->val);
        }
        //快慢指针找到对应的中间节点
        ListNode *pre=NULL;
        ListNode *mid=head;
        ListNode *fast=head;
        while(fast!=NULL&&fast->next!=NULL){
            pre=mid;
            mid=mid->next;
            fast=fast->next->next;
        }
        root=new TreeNode(mid->val);
        pre->next=NULL;//将中间节点以前的断开
        root->left=sortedListToBST(head); //递归左侧
        root->right=sortedListToBST(mid->next);//递归右侧
        
        return root;
    
    }
    
    TreeNode *arraytoTree(vector<int>&ivec,int start,int end){
        int mid=start+(end-start)/2;
        TreeNode *root=new TreeNode(ivec[mid]);
        if(start==end){
            return root;   
        }
        if(start<=mid-1){
            root->left=arraytoTree(ivec,start,mid-1);
        }
        if(mid+1<=end){
            root->right=arraytoTree(ivec,mid+1,end);
        }
        
        return root;
    }
    
    void addnode(TreeNode * &root,int value){
        if(root==NULL){
            root=new TreeNode(value);
        }else if(root->val>value){
            addnode(root->left,value);
        }else if(root->val<value){
            addnode(root->right,value);
        }
    }
};