滑动窗口+前缀和-8--LC930.和相同的二元子数组

class Solution(object):
    def numSubarraysWithSum(self, nums, goal):
        """
        :type nums: List[int]
        :type goal: int
        :rtype: int
        """
        # 1.前缀和
        presum = 0
        adict = {0:1}
        count = 0
        for i in range(len(nums)):
            presum += nums[i]
            if presum - goal in adict:
                count += adict[presum-goal]
            if presum not in adict:
                adict[presum] = 1
            else:
                adict[presum] += 1
        return count

class Solution(object):
    def numSubarraysWithSum(self, nums, goal):
        """
        :type nums: List[int]
        :type goal: int
        :rtype: int
        """
		# 数组中只有0和1，和就是1的个数
        def leGoal(nums, goal):
            if goal < 0:
                return 0
            start = 0
            one_count = 0
            count = 0
            for end in range(len(nums)):
                if nums[end] == 1:
                    one_count += 1
                while one_count > goal:
                    if nums[start] == 1:
                        one_count -= 1
                    start += 1
                count += end-start+1
            return count
        return leGoal(nums, goal)-leGoal(nums, goal-1)