LeetCode 930. Binary Subarrays With Sum

原题链接在这里：https://leetcode.com/problems/binary-subarrays-with-sum/

题目：

In an array A of 0s and 1s, how many non-empty subarrays have sum S?

Example 1:

Input: A = [1,0,1,0,1], S = 2
Output: 4
Explanation:
The 4 subarrays are bolded below:
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]

Note:

1. A.length <= 30000
2. 0 <= S <= A.length
3. A[i] is either 0 or 1.

题解：

Maintian the count of sum from 0 to i.

If sum >= S, then res += count[sum-S].

Because, there must be t, t<i, sum from 0 to t is sum-S, from t to i is S. Then count of S is count of sum-S.

Time Complexity: O(n). n = A.length.

Space: O(n).

AC Java:

 class Solution {
     public int numSubarraysWithSum(int[] A, int S) {
         if(A == null || A.length == 0){
             return 0;
         }

         int [] count = new int[A.length+1];
         count[0] = 1;
         int sum = 0;
         int res = 0;
         for(int a : A){
             sum += a;
             if(sum >= S){
                 res += count[sum-S];
             }

             count[sum]++;
         }

         return res;
     }
 }