leetcode669. 修剪二叉搜索树

注意：×

1. 刚开始想不通为什么啥操作都没有，直接return就好了，仔细思考一下，其实null的操作就是把这个分支给去除掉了

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

        if (root.val > high){
            return trimBST(root.left, low, high);
        }
        
        root.left = trimBST(root.left, low, high);
        root.right = trimBST(root.right, low, high);
        
        return root;
        
    }

leetcode108.将有序数组转换为二叉搜索树

注意：√

1. 一遍过，感觉和之前的数组构建二叉树很像，Labuladong书里面肯定有，不记得随想录里面有没有这个

    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums.length == 0){
            return null;
        }

        return build(nums, 0, nums.length-1);
    }

    private TreeNode build(int[] nums, int lo, int hi) {
        if (lo>hi){
            return null;
        }
        int index = lo+(hi-lo)/2;

        TreeNode root = new TreeNode(nums[index]);
        root.left = build(nums, lo, index-1);
        root.right = build(nums, index+1, hi);
        
        return root;
    }

leetcode538. 把二叉搜索树转换为累加树

注意：√

1. 之前看过Labuladong书中内容，先右后左的遍历顺序+外部sum解决

    int sum = 0;

    public TreeNode convertBST(TreeNode root) {
        traverse(root);
        return root;
    }

    private void traverse(TreeNode root) {
        if (root == null){
            return;
        }
        
        traverse(root.right);
        
        sum = sum+root.val;
        root.val = sum;
        
        traverse(root.left);
        
        return;
        
    }