Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order. A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does…

在二维网格 grid 上，有 4 种类型的方格：

Given an integer n, return all the structurally unique BST's (binary search trees), which has exactly n nodes of unique values from 1 to n. Return the answer in any order.   Example 1: Input: n = 3 Output: [[1,null,2,null,3],[1,null,3,2],[2,1,3],[3,1,null…

给你一个正整数 n ，请你返回 n 的 惩罚数 。 n 的 惩罚数 定义为所有满足以下条件 i 的数的平方和：