题目：

题解：

class NumArray {private int[] tree;private int[] nums;public NumArray(int[] nums) {this.tree = new int[nums.length + 1];this.nums = nums;for (int i = 0; i < nums.length; i++) {add(i + 1, nums[i]);}}public void update(int index, int val) {add(index + 1, val - nums[index]);nums[index] = val;}public int sumRange(int left, int right) {return prefixSum(right + 1) - prefixSum(left);}private int lowBit(int x) {return x & -x;}private void add(int index, int val) {while (index < tree.length) {tree[index] += val;index += lowBit(index);}}private int prefixSum(int index) {int sum = 0;while (index > 0) {sum += tree[index];index -= lowBit(index);}return sum;}
}