leetcode 303. Range Sum Query - Immutable（前缀和）

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.

前缀和的应用，很简单。

class NumArray {
public:
int n;
vector<int> s;
NumArray(vector<int> &nums) {
n = nums.size();
if(n==0)return;

s.push_back(nums[0]);
for(int i=1;i<n;i++){
s.push_back(s.back()+nums[i]);
}
}

int sumRange(int i, int j) {
if(i>j)return 0;
if(i==0) return s[j];
return s[j]-s[i-1];
}
};

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