Contains Duplicate II
描述
Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the difference between i and j is at most k.
分析
维护一个HashMap, key为整数，value为下标，将数组中的元素不断添加进这个HashMap, 碰到重复时，计算二者的下标距离，如果距离小于或等于k，则返回true, 如果直到数组扫描完，距离都大于k，则返回false。