Minimum Size Subarray Sum

Using a slide window. For each i we can find the shortest subarray that the sum of elements is greater than s. Instead of enumerate all subarray, we using a slide window and a variable sum to keep track of the partial sum.

public class Solution {
    public int minSubArrayLen(int s, int[] nums) {
        int sum = 0;
        int l = -1;
        int min = nums.length + 1;
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            while (sum >= s) {
                min = Math.min(min, i - l);
                sum -= nums[++l];
            }
        }
        if (min > nums.length) return 0;
        else return min;
    }
}

This solution is correct based on the following observasion:

1. if for i, the shortest subarray starts from l, then if there exists such a subarray ends at i + 1 and shorter than that for i, then the subarray must start after l.
2. if the sum of all elements in the array is less than s, min will never be modified. So if at last min > nums.length we should return 0.