形式汇总：
206. Reverse Linked List
92. Reverse Linked List II:Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string.
344. Reverse String:Given s = "hello", return "olleh".
151. Reverse Words in a String：For example,Given s = "the sky is blue",return "blue is sky the". 去掉空格后，用string builder函数
541. Reverse String II:Given a string and an integer k, you need to reverse the first k characters for every 2k characters counting from the start of the string. If there are less than k characters left, reverse all of them.
557. Reverse Words in a String III：Given a string, you need to reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.
345. Reverse Vowels of a String
7. Reverse Integer :120-21：用＊10，％10
190. Reverse Bits：位运算
189. Rotate Array:with n = 7 and k = 3, the array [1,2,3,4,5,6,7] is rotated to [5,6,7,1,2,3,4].
493. Reverse Pairs：i < j and nums[i] > 2*nums[j].