209. Minimum Size Subarray Sum

题目：求数组中和大于某数的最小子数组的长度
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.

Example:

Input: s = 7, nums = [2,3,1,2,4,3]
Output: 2
Explanation: the subarray [4,3] has the minimal length under the problem constraint.

又是一题巧用更新法的题目了，当得到大于某数的子数组的时候，记录下长度，并将总和减去最前面的一个值，这样就可以继续往下寻找，直到遍历结束

class Solution {
public:
    int minSubArrayLen(int s, vector& nums) {
        int n = nums.size(), start = 0, sum = 0, minlen = INT_MAX;
        for (int i = 0; i < n; i++) { 
            sum += nums[i]; 
            while (sum >= s) {
                minlen = min(minlen, i - start + 1);
                sum -= nums[start++];
            }
        }
        return minlen == INT_MAX ? 0 : minlen;
    }
};