LeetCode 209 Minimum Size Subarray Sum

Problem:

Given an array of n positive integers and a positive integer s, find the minimal length of a 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.

Summary:

找到数组中和大于目标值s的最短子序列。

Solution:

用两个指针分别代表当前子序列的开始和结尾，若计算出的sum小于s，则end++，否则start--，每计算出一个更短的子序列，更新res值。

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