Formatted question description: https://leetcode.ca/all/1508.html
1508. Range Sum of Sorted Subarray Sums (Medium)
Given the array nums consisting of n positive integers. You computed the sum of all non-empty continous subarrays from the array and then sort them in non-decreasing order, creating a new array of n * (n + 1) / 2 numbers.
Return the sum of the numbers from index left to index right (indexed from 1), inclusive, in the new array. Since the answer can be a huge number return it modulo 10^9 + 7.
Example 1:
Input: nums = [1,2,3,4], n = 4, left = 1, right = 5 Output: 13 Explanation: All subarray sums are 1, 3, 6, 10, 2, 5, 9, 3, 7, 4. After sorting them in non-decreasing order we have the new array [1, 2, 3, 3, 4, 5, 6, 7, 9, 10]. The sum of the numbers from index le = 1 to ri = 5 is 1 + 2 + 3 + 3 + 4 = 13.
Example 2:
Input: nums = [1,2,3,4], n = 4, left = 3, right = 4 Output: 6 Explanation: The given array is the same as example 1. We have the new array [1, 2, 3, 3, 4, 5, 6, 7, 9, 10]. The sum of the numbers from index le = 3 to ri = 4 is 3 + 3 = 6.
Example 3:
Input: nums = [1,2,3,4], n = 4, left = 1, right = 10 Output: 50
Constraints:
1 <= nums.length <= 10^3nums.length == n1 <= nums[i] <= 1001 <= left <= right <= n * (n + 1) / 2
Solution 1. Prefix Sum
Compute the prefix sum array using pre. Use pre to get the array of all subarray sums – sum. Sort sum array then add from sum[left - 1] to sum[right - 1].
// OJ: https://leetcode.com/problems/range-sum-of-sorted-subarray-sums/
// Time: O(N^2 * log(N^2))
// Space: O(N^2)
class Solution {
public:
int rangeSum(vector<int>& A, int N, int left, int right) {
vector<int> sum;
long ans = 0, mod = 1e9+7;
for (int i = 0; i < N; ++i) {
int pre = 0;
for (int j = i; j < N; ++j) {
pre += A[j];
sum.push_back(pre);
}
}
sort(begin(sum), end(sum));
for (int i = left; i <= right; ++i) ans = (ans + sum[i - 1]) % mod;
return ans;
}
};
Solution 2. Heap
// OJ: https://leetcode.com/problems/range-sum-of-sorted-subarray-sums/
// Time: O(right * logN)
// Space: O(N)
// Ref: https://leetcode.com/problems/range-sum-of-sorted-subarray-sums/discuss/730511/C++-priority_queue-solution
class Solution {
public:
int rangeSum(vector<int>& A, int N, int left, int right) {
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> q; // partial sum, next index
for (int i = 0; i < N; ++i) q.emplace(A[i], i + 1);
long ans = 0, mod = 1e9+7;
for (int i = 1; i <= right; ++i) {
auto p = q.top();
q.pop();
if (i >= left) ans = (ans + p.first) % mod;
if (p.second < N) {
p.first += A[p.second++];
q.push(p);
}
}
return ans;
}
};
Solution 3. Binary Search + Sliding Window
// OJ: https://leetcode.com/problems/range-sum-of-sorted-subarray-sums/
// Time: O(Nlog(sum(A)))
// Space: O(N)
// Ref: https://leetcode.com/problems/range-sum-of-sorted-subarray-sums/discuss/733047/Python-Binary-Search-Time-O(NlogSum(A))
class Solution {
vector<int> B, C; // B is partial_sum array of A. C is partial_sum array of B.
int N;
int countSumUnder(int score) { // Use sliding window to cound the partial sums that are smaller or equal to `score`.
int ans = 0, i = 0;
for (int j = 0; j < N + 1; ++j) {
while (B[j] - B[i] > score) ++i;
ans += j - i;
}
return ans;
}
int sumKSums(int k) { // Use sliding window to get the sum of the k smallest items in all the subarray sums.
int score = kthScore(k), ans = 0, i = 0;
for (int j = 0; j < N + 1; ++j) {
while (B[j] - B[i] > score) ++i;
ans += B[j] * (j - i + 1) - (C[j] - (i ? C[i - 1] : 0));
}
return ans - (countSumUnder(score) - k) * score;
}
int kthScore(int k) { // use binary search to find the kth smallest sum in all subarray sums.
int L = 0, R = B[N];
while (L < R) {
int M = (L + R) / 2;
if (countSumUnder(M) < k) ++L;
else R = M;
}
return L;
}
public:
int rangeSum(vector<int>& A, int n, int left, int right) {
N = n;
B.assign(N + 1, 0);
C.assign(N + 1, 0);
for (int i = 0; i < N; ++i) {
B[i + 1] = B[i] + A[i];
C[i + 1] = C[i] + B[i + 1];
}
return sumKSums(right) - sumKSums(left - 1);
}
};
Java
class Solution {
public int rangeSum(int[] nums, int n, int left, int right) {
final int MODULO = 1000000007;
int length = nums.length;
int sumsLength = length * (length + 1) / 2;
int[] sums = new int[sumsLength];
int index = 0;
for (int i = 0; i < length; i++) {
int sum = 0;
for (int j = i; j < length; j++) {
sum += nums[j];
sums[index++] = sum;
}
}
Arrays.sort(sums);
int rangeSum = 0;
for (int i = left - 1; i < right; i++)
rangeSum = (rangeSum + sums[i]) % MODULO;
return rangeSum;
}
}