代码随想录 -- day23 -- ● 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

class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if (root == nullptr ) return nullptr;
        if (root->val < low) {
            TreeNode* right = trimBST(root->right, low, high); // 寻找符合区间[low, high]的节点
            return right;
        }
        if (root->val > high) {
            TreeNode* left = trimBST(root->left, low, high); // 寻找符合区间[low, high]的节点
            return left;
        }
        root->left = trimBST(root->left, low, high); // root->left接入符合条件的左孩子
        root->right = trimBST(root->right, low, high); // root->right接入符合条件的右孩子
        return root;
    }
};

08.将有序数组转换为二叉搜索树

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

538.把二叉搜索树转换为累加树 

class Solution {
private:
    int pre = 0; // 记录前一个节点的数值
    void traversal(TreeNode* cur) { // 右中左遍历
        if (cur == NULL) return;
        traversal(cur->right);
        cur->val += pre;
        pre = cur->val;
        traversal(cur->left);
    }
public:
    TreeNode* convertBST(TreeNode* root) {
        pre = 0;
        traversal(root);
        return root;
    }
};