【每日算法】和相同的二元子数组 :「前缀和 + 哈希表」&「双指针」 ｜Python 主题月

输入：nums = [1,0,1,0,1], goal = 2
输出：4
解释：
如下面黑体所示，有 4 个满足题目要求的子数组：
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
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输入：nums = [0,0,0,0,0], goal = 0
输出：15
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class Solution {
    public int numSubarraysWithSum(int[] nums, int t) {
        int n = nums.length;
        int[] sum = new int[n + 1];
        for (int i = 1; i <= n; i++) sum[i] = sum[i - 1] + nums[i - 1];
        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, 1);
        int ans = 0;
        for (int i = 0; i < n; i++) {
            int r = sum[i + 1], l = r - t;
            ans += map.getOrDefault(l, 0);
            map.put(r, map.getOrDefault(r, 0) + 1);
        }
        return ans;
    }
}
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class Solution:
    def numSubarraysWithSum(self, nums: List[int], goal: int) -> int:
        n = len(nums)
        presum = [0] + list(accumulate(nums))
        hashmap = defaultdict(int, {0:1})
        ans = 0
        for i in range(n):
            r = presum[i+1]
            l = r - goal
            ans += hashmap[l]
            hashmap[r] += 1
        return ans
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class Solution {
    public int numSubarraysWithSum(int[] nums, int t) {
        int n = nums.length;
        int ans = 0;
        for (int r = 0, l1 = 0, l2 = 0, s1 = 0, s2 = 0; r < n; r++) {
            s1 += nums[r];
            s2 += nums[r];
            while (l1 <= r && s1 > t) s1 -= nums[l1++];
            while (l2 <= r && s2 >= t) s2 -= nums[l2++];
            ans += l2 - l1;
        }
        return ans;
    }
}
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class Solution:
    def numSubarraysWithSum(self, nums: List[int], goal: int) -> int:
        n = len(nums)
        ans = l1 = l2 = s1 = s2 = 0
        for r in range(n):
            s1 += nums[r]
            s2 += nums[r]
            while l1 <= r and s1 > goal:
                s1 -= nums[l1]
                l1 += 1
            while l2 <= r and s2 >= goal:
                s2 -= nums[l2]
                l2 += 1
            ans += l2 - l1
        return ans
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