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:

1. You may assume that the array does not change.
2. 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);