g0301_0400.s0327_count_of_range_sum.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0301_0400.s0327_count_of_range_sum;

// #Hard #Array #Binary_Search #Ordered_Set #Divide_and_Conquer #Segment_Tree #Binary_Indexed_Tree
// #Merge_Sort

public class Solution {
    public int countRangeSum(int[] nums, int lower, int upper) {
        int n = nums.length;
        long[] sums = new long[n + 1];
        for (int i = 0; i < n; i++) {
            sums[i + 1] = sums[i] + nums[i];
        }
        return countWhileMergeSort(sums, 0, n + 1, lower, upper);
    }

    private int countWhileMergeSort(long[] sums, int start, int end, int lower, int upper) {
        if (end - start <= 1) {
            return 0;
        }
        int mid = (start + end) / 2;
        int count =
                countWhileMergeSort(sums, start, mid, lower, upper)
                        + countWhileMergeSort(sums, mid, end, lower, upper);
        int j = mid;
        int k = mid;
        int t = mid;
        long[] cache = new long[end - start];
        int r = 0;
        for (int i = start; i < mid; i++) {
            while (k < end && sums[k] - sums[i] < lower) {
                k++;
            }
            while (j < end && sums[j] - sums[i] <= upper) {
                j++;
            }
            while (t < end && sums[t] < sums[i]) {
                cache[r++] = sums[t++];
            }
            cache[r] = sums[i];
            count += j - k;
            r++;
        }
        System.arraycopy(cache, 0, sums, start, t - start);
        return count;
    }
}