Day 23 | 669. Trim a BST | 108. Convert Sorted Array to BST | 538. Convert BST to Greater Tree

Day 1 | 704. Binary Search | 27. Remove Element | 35. Search Insert Position | 34. First and Last Position of Element in Sorted Array
Day 2 | 977. Squares of a Sorted Array | 209. Minimum Size Subarray Sum | 59. Spiral Matrix II
Day 3 | 203. Remove Linked List Elements | 707. Design Linked List | 206. Reverse Linked List
Day 4 | 24. Swap Nodes in Pairs| 19. Remove Nth Node From End of List| 160.Intersection of Two Lists
Day 6 | 242. Valid Anagram | 349. Intersection of Two Arrays | 202. Happy Numbe | 1. Two Sum
Day 7 | 454. 4Sum II | 383. Ransom Note | 15. 3Sum | 18. 4Sum
Day 8 | 344. Reverse String | 541. Reverse String II | 替换空格 | 151.Reverse Words in a String | 左旋转字符串
Day 9 | 28. Find the Index of the First Occurrence in a String | 459. Repeated Substring Pattern
Day 10 | 232. Implement Queue using Stacks | 225. Implement Stack using Queue
Day 11 | 20. Valid Parentheses | 1047. Remove All Adjacent Duplicates In String | 150. Evaluate RPN
Day 13 | 239. Sliding Window Maximum | 347. Top K Frequent Elements
Day 14 | 144.Binary Tree Preorder Traversal | 94.Binary Tree Inorder Traversal| 145.Binary Tree Postorder Traversal
Day 15 | 102. Binary Tree Level Order Traversal | 226. Invert Binary Tree | 101. Symmetric Tree
Day 16 | 104.MaximumDepth of BinaryTree| 111.MinimumDepth of BinaryTree| 222.CountComplete TreeNodes
Day 17 | 110. Balanced Binary Tree | 257. Binary Tree Paths | 404. Sum of Left Leaves
Day 18 | 513. Find Bottom Left Tree Value | 112. Path Sum | 105&106. Construct Binary Tree
Day 20 | 654. Maximum Binary Tree | 617. Merge Two Binary Trees | 700.Search in a Binary Search Tree
Day 21 | 530. Minimum Absolute Difference in BST | 501. Find Mode in Binary Search Tree | 236. Lowes
Day 22 | 235. Lowest Common Ancestor of a BST | 701. Insert into a BST | 450. Delete Node in a BST

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LeetCode 669. Trim a Binary Search Tree

Question Link

class Solution {
    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);
        // The value of root in the range
        root.left = trimBST(root.left, low, high);
        root.right = trimBST(root.right, low, high);
        return root;
    }
}

LeetCode 108. Convert Sorted Array to Binary Search Tree

Question Link

class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return traversal(nums, 0, nums.length - 1);
    }

    TreeNode traversal(int[] nums, int low, int high){
        if(low > high)
            return null;
        int middle = low + (high - low)/2;
        TreeNode node = new TreeNode(nums[middle]);
        node.left = traversal(nums, low, middle-1);
        node.right = traversal(nums, middle+1, high);
        return node;
    }
}

• low + ((high - low) >> 1) is equals to (low+high)/2. The first one can avoid cross-border problem when high and low are bigest int.

LeetCode 538. Convert BST to Greater Tree

Question Link

class Solution {
    int pre = 0;    // record the value of previous node
    public TreeNode convertBST(TreeNode root) {
        traversal(root);
        return root;
    }

    void traversal(TreeNode root){
        if(root == null)
            return;

        traversal(root.right);
        pre += root.val;
        root.val = pre;
        traversal(root.left);
    }
}