209. Minimum Size Subarray Sum

class Solution(object):
    def minSubArrayLen(self, s, nums):
        """
        :type s: int
        :type nums: List[int]
        :rtype: int
        """
        
        # two pointers
        if len(nums) == 0:
            return 0
        
        sum = 0
        i = 0
        # fast pointer j
        j = 0
        res = len(nums) + 1
        while sum < s:
            sum += nums[j]
            j += 1
            
            # to enter the while loop below, sum is greater than s, now let‘s use another pointer i(slow) pointer to find the intervals between i and j.  
            
            while sum >= s:
                # if i and j still have elements among them
                if  res > j - i:
                    res = j - i
                sum -= nums[i]
                i += 1
                
                
            if j == len(nums):
                break
        
        # if sum > s at the very beginning, then the program will not enter the first while loop. Thus 0 will be returned. 
        if res > len(nums):
            return 0
        
        return res