代码随想录算法训练营第20天| 669. 修剪二叉搜索树 ，108.将有序数组转换为二叉搜索树，538.把二叉搜索树转换为累加树

669. 修剪二叉搜索树

文章链接：https://programmercarl.com/0669.修剪二叉搜索树.html
题目链接：https://leetcode.cn/problems/trim-a-binary-search-tree/description/

class Solution {
public:

    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if(root==NULL) return NULL;
        if(root->val<low){ //如果小于规定范围
            TreeNode* node=trimBST(root->right,low,high);
            return node;
        }
        else if(root->val>high){
            TreeNode* node=trimBST(root->left,low,high);
            return node;
        }
        else{
            root->left=trimBST(root->left,low,high);
            root->right=trimBST(root->right,low,high);
            return root;
        }
       
    }
};

108.将有序数组转换为二叉搜索树

文章链接：https://programmercarl.com/0108.将有序数组转换为二叉搜索树.html
题目链接：https://leetcode.cn/problems/convert-sorted-array-to-binary-search-tree/description/

class Solution {
public:
    TreeNode* traversal(vector<int>& nums,int left,int right){
        if(left>right) return NULL;
        int divIndex=left+(right-left)/2;
        TreeNode* node=new TreeNode(nums[divIndex]);
        node->left=traversal(nums,left,divIndex-1);
        node->right=traversal(nums,divIndex+1,right);
        return node;
    }
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return traversal(nums,0,nums.size()-1);
    }
};

538.把二叉搜索树转换为累加树

文章链接：https://programmercarl.com/0538.把二叉搜索树转换为累加树.html
题目链接：https://leetcode.cn/problems/convert-bst-to-greater-tree/description/

class Solution {
public:
    int sum=0;
    void traversal(TreeNode* root){
        //使用右中左的顺序
        if(root==NULL) return;
        traversal(root->right);
        sum+=root->val;
        root->val=sum;
        traversal(root->left);
    }
    TreeNode* convertBST(TreeNode* root) {
        traversal(root);
        return root;
    }
};