2、在介绍尺取法的使用之前，先看一个典型的实例leetcode209. minimum size subarray sum：given an array of n positive integers and a positive integer s, find the minimal length of a contiguous subarray of which the sum ≥ s. if there isn't one, return 0 instead.for example, given the array [2,3,1,2,4,3] and s = 7,the subarray [4,3] has the minimal length under the problem constraint.