Leetcode前缀和问题集锦

相关题目
leetcode 560 subarraySum

int subarraySum(vector<int>& nums, int k) {
      unordered_map<int, int> hash;
      int count = 0, psum = 0;
      hash[0] = 1;
      for(int i = 0; i < nums.size(); i++){
          psum += nums[i];
          count += hash[psum - k];
          hash[psum]++;
      }
      return count;
  }

leetcode 974 subarraysDivByK

int subarraysDivByK(vector<int>& nums, int k) {
	 int ans = 0, n = nums.size(), curSum = 0;
	 unordered_map<int, int> hash;
	 hash[0] = 1;
	 int mod;
	 for(int i = 0; i < n; i++){
	     curSum += nums[i];
	     mod = (curSum % k + k) % k;
	     ans += hash[mod];
	     hash[mod]++;
	 }
	 return ans;
}

leetcode 1248 numberOfSubarrays

int numberOfSubarrays(vector<int>& nums, int k) {
      unordered_map<int, int> m;
      const int n = nums.size();
      int rst = 0;
      int acc = 0;
      m[0] = 1;
      for (int i = 0; i < n; ++i) {
          acc += (nums[i]%2);
          rst += m[acc-k];
          m[acc]++;
      }
      return rst;
  }

leetcode 1588 sumOddLengthSubarrays

int sumOddLengthSubarrays(vector<int>& nums) {
      int ans = 0, n = nums.size();
      vector<int> preSum(n + 1, 0);
      for(int i = 0; i < n; i++){
          preSum[i+1] = preSum[i] + nums[i];
      }
      for(int i = 0; i < n; i++){
          for(int j = i + 1; j <= n; j+=2){
              ans += preSum[j] - preSum[i];
          }
      }
      return ans;
  }