[LeetCode] Range Sum Query - Mutable

This problem requires a new data structure --- Segment Tree. You may use this GeeksforGeeks article to get some ideas of it. However, the code in this article may be too verbose. To solve this problem using segment tree, 2guotou has posted a very nice Java code, which is rewritten below in C++. I try to conform to the LeetCode traditions of defining structures and classes.

struct SegmentTreeNode {
int s, e, sum;
SegmentTreeNode* left;
SegmentTreeNode* right;
SegmentTreeNode(int _s, int _e) : s(_s), e(_e), sum(0), left(NULL), right(NULL) {}
};

class SegmentTree {
public:
SegmentTree(vector<int>& nums) {
int n = nums.size();
root = buildST(nums, 0, n - 1);
}

void update(int i, int val) {
update(root, i, val);
}

int sumRange(int i, int j) {
return sumRange(root, i, j);
}
private:
SegmentTreeNode* root;
SegmentTreeNode* buildST(vector<int>& nums, int s, int e) {
if (s > e) return NULL;
else {
SegmentTreeNode* res = new SegmentTreeNode(s, e);
if (s == e) res->sum = nums[s];
else {
int m = s + (e - s) / 2;
res->left = buildST(nums, s, m);
res->right = buildST(nums, m + 1, e);
res->sum = res->left->sum + res->right->sum;
}
return res;
}
}
void update(SegmentTreeNode* root, int i, int val) {
if (root->s == root->e) root->sum = val;
else {
int m = root->s + (root->e - root->s) / 2;
if (i <= m) update(root->left, i, val);
else update(root->right, i, val);
root->sum = root->left->sum + root->right->sum;
}
}
int sumRange(SegmentTreeNode* root, int s, int e) {
if (root->s == s && root->e == e) return root->sum;
else {
int m = root->s + (root->e - root->s) / 2;
if (e <= m) return sumRange(root->left, s, e);
else if (s >= m + 1) return sumRange(root->right, s, e);
else return sumRange(root->left, s, m) + sumRange(root->right, m + 1, e);
}
}
};

class NumArray {
public:
NumArray(vector<int>& nums) {
st = new SegmentTree(nums);
}

void update(int i, int val) {
st->update(i, val);
}

int sumRange(int i, int j) {
return st->sumRange(i, j);
}
private:
SegmentTree* st;
};

// Your NumArray object will be instantiated and called as such:
// NumArray numArray(nums);
// numArray.sumRange(0, 1);
// numArray.update(1, 10);
// numArray.sumRange(1, 2);