[LeetCode] Minimum Size Subarray Sum

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.

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Credits:
Special thanks to @Freezen for adding this problem and creating all test cases.

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