?LeetCode刷題實戰(zhàn)307：區(qū)域和檢索 - 數(shù)組可修改

輸入：
["NumArray", "sumRange", "update", "sumRange"]
[[[1, 3, 5]], [0, 2], [1, 2], [0, 2]]
輸出：
[null, 9, null, 8]

解釋：
NumArray numArray = new NumArray([1, 3, 5]);
numArray.sumRange(0, 2); // 返回 9 ，sum([1,3,5]) = 9
numArray.update(1, 2); // nums = [1,2,5]
numArray.sumRange(0, 2); // 返回 8 ，sum([1,2,5]) = 8

int[] tree;
int n;
public NumArray(int[] nums) {
    if (nums.length > 0) {
        n = nums.length;
        tree = new int[n * 2];
        buildTree(nums);
    }
}
private void buildTree(int[] nums) {
    for (int i = n, j = 0; i < 2 * n; i++, j++)
        tree[i] = nums[j];
    for (int i = n - 1; i > 0; --i)
        tree[i] = tree[i * 2] + tree[i * 2 + 1];
}