力扣1248.统计优美子数组

力扣1248.统计优美子数组

• 同930.

哈希表法

• 求前缀和

  class Solution {
  public:
      int numberOfSubarrays(vector<int>& nums, int k) {
          int n = nums.size();
          unordered_map<int,int> cnt;
          int res=0,sum=0;
          for(int i=0,j=0;i<n;i++)
          {
              cnt[sum] ++;
              if(nums[i] & 1) sum ++;
              res += cnt[sum - k];
          }
          return res;
      }
  };

多指针法

• 用两个左指针卡左边可取的区间范围

  class Solution {
  public:
      int numberOfSubarrays(vector<int>& nums, int k) {
          int n = nums.size();
          int res=0,s1=0,s2=0;
          for(int i=0,j1=0,j2=0;i<n;i++)
          {
              int x = (nums[i] & 1);
              s1 += x;
              s2 += x;
              //s1 --> s1 < k
              while(j1 <= i && s1 >= k) s1 -= (nums[j1++] & 1);
              //s2 --> s2 == k
              while(j2 <= i && s2 > k) s2 -= (nums[j2++] & 1);
              //此时j1的位置比j2靠右
              res += j1 - j2;
          }
          return res;
      }
  };