题目：

题解：

class NumArray {
private:
    vector<int> tree;
    vector<int> &nums;

    int lowBit(int x) {
        return x & -x;
    }

    void add(int index, int val) {
        while (index < tree.size()) {
            tree[index] += val;
            index += lowBit(index);
        }
    }

    int prefixSum(int index) {
        int sum = 0;
        while (index > 0) {
            sum += tree[index];
            index -= lowBit(index);
        }
        return sum;
    }

public:
    NumArray(vector<int>& nums) : tree(nums.size() + 1), nums(nums) {
        for (int i = 0; i < nums.size(); i++) {
            add(i + 1, nums[i]);
        }
    }

    void update(int index, int val) {
        add(index + 1, val - nums[index]);
        nums[index] = val;
    }

    int sumRange(int left, int right) {
        return prefixSum(right + 1) - prefixSum(left);
    }
};