LeetCode Range Sum Query - Immutable

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:

Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:

You may assume that the array does not change.
There are many calls to sumRange function.

看似简单，但直接在函数内直接求和会超时，题目提示数组是变的，所以我们可以做预处理，然后再求和函数中直接得出结果。题目是求和查询也有直接查出求和结果的意思。

class NumArray {
public:
NumArray(vector<int> &nums) {
num.clear();
if(nums.size()  != 0){
num.push_back(nums[0]);
for(int i = 1; i < nums.size(); i++)
num.push_back(num[i-1]+nums[i]);
}
}

int sumRange(int i, int j) {
if(i <= 0)
return num[j];
return num[j]-num[i-1];
}
private:
vector<int> num;
};

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