Brush questions in summer -day3

1.303. Area and retrieval - The array is immutable

 

class NumArray {
public:
    vector<int> sums;

    NumArray(vector<int>& nums) {
        int n = nums.size();
        sums.resize(n + 1);
        for (int i = 0; i < n; i++) {
            sums[i + 1] = sums[i] + nums[i];
        }
    }

    int sumRange(int i, int j) {
        return sums[j + 1] - sums[i];
    }
};

2.1588. The sum of all odd length subarrays  

 

class Solution {
public:
    int sumOddLengthSubarrays(vector<int>& arr) {
        int sum = 0;
        int n = arr.size();
        for (int start = 0; start < n; start++) {
            for (int length = 1; start + length <= n; length += 2) {
                int end = start + length - 1;
                for (int i = start; i <= end; i++) {
                    sum += arr[i];
                }
            }
        }
        return sum;
    }
};

3.930. And the same binary subarray  

 

 

 

class Solution {
public:
    int numSubarraysWithSum(vector<int>& nums, int goal) {
        int sum = 0;
        unordered_map<int, int> cnt;
        int ret = 0;
        for (auto& num : nums) {
            cnt[sum]++;
            sum += num;
            ret += cnt[sum - goal];
        }
        return ret;
    }
};