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);}
};