669. 修剪二叉搜索树

class Solution {
    public TreeNode trimBST(TreeNode root, int low, int high) {
        if(root == null)return null;
        if(root.val > high)return trimBST(root.left, low, high);
        if(root.val < low)return trimBST(root.right, low, high);

        root.right = trimBST(root.right, low, high);
        root.left = trimBST(root.left, low, high);
        return root;
    }
}

108. 将有序数组转换为二叉搜索树

class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        TreeNode root = traverse(nums, 0, nums.length - 1);
        return root;
    }
    private TreeNode traverse(int[] nums, int left, int right){
        if(left > right)return null;
        int mid = left + ((right - left) / 2);
        TreeNode root = new TreeNode(nums[mid]);
        root.left = traverse(nums, left, mid - 1);
        root.right = traverse(nums, mid + 1, right);
        return root;
    }
}

538. 把二叉搜索树转换为累加树

能比root.val大的值都在root的右边，所以先从右边开始累加。

class Solution {
    public TreeNode convertBST(TreeNode root) {
        traverse(root);
        return root;
    }
    int sum = 0; // 记录累加和
    private void traverse(TreeNode root){
        if(root == null){
            return;
        }
        traverse(root.right);
        sum += root.val; // 维护累加和
        root.val = sum;
        traverse(root.left);
    }
}