leetcode_c++:Divide and Conquer：Count of Range Sum(327)

Given an integer array nums, return the number of range sums that lie in [lower, upper] inclusive.

Range sum S(i, j) is defined as the sum of the elements in nums between indices i and j (i ≤ j), inclusive.

Note:

A naive algorithm of O(n2) is trivial. You MUST do better than that.

Example:

Given nums = [-2, 5, -1], lower = -2, upper = 2,

Return 3.

The three ranges are : [0, 0], [2, 2], [0, 2] and their respective sums are: -2, -1, 2.

typedef long long LL;
struct SegmentTreeNode {
LL L, R;
int cnt;
SegmentTreeNode *left, *right;
SegmentTreeNode(LL L, LL R) :L(L), R(R), cnt(0), left(NULL), right(NULL) {}
};

class SegmentTree {
SegmentTreeNode * root;
SegmentTreeNode * buildTree(vector<LL> &nums, int L, int R) {
if (L > R) return NULL;
SegmentTreeNode * root = new SegmentTreeNode(nums[L], nums[R]);
if (L == R) return root;
int mid = (L + R) >> 1;
root->left = buildTree(nums, L, mid);
root->right = buildTree(nums, mid + 1, R);
return root;
}

void update(SegmentTreeNode * root, LL val) {
if (root && root->L <= val &&  val <= root->R) {
root->cnt++;
update(root->left, val);
update(root->right, val);
}
}

int sum(SegmentTreeNode * root, LL L, LL R) {
if (!root || root->R < L ||  R < root->L ) return 0;
if (L <= root->L  && root->R <= R) return root->cnt;
return sum(root->left, L, R) + sum(root->right, L, R);
}

public:
SegmentTree(vector<LL> &nums, int L, int R) { root = buildTree(nums, L, R); }

int sum(LL L, LL R) {
return sum(root, L, R);
}

void update(LL val) {
update(root, val);
}
};

class Solution {
public:
int countRangeSum(vector<int>& nums, int lower, int upper) {
if (nums.size() == 0) return 0;
vector<LL> sum_array (nums.size(),0);
sum_array[0] = nums[0];
for (int i = 1; i < sum_array.size(); i++) {
sum_array[i] = nums[i] + sum_array[i - 1];
}
LL sum = sum_array[sum_array.size() - 1];
sort(sum_array.begin(), sum_array.end());
auto t = unique(sum_array.begin(), sum_array.end());
SegmentTree tree(sum_array, 0, t - sum_array.begin() - 1);
int ans = 0;
for (int i = nums.size() - 1; i >= 0; i--) {
tree.update(sum);
sum -= nums[i];
ans += tree.sum(lower + sum,upper + sum);
}
return ans;
}
};