303. Range Sum Query - Immutable

题目来源【Leetcode】

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.

求数组某个区间的和，代码如下:

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

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

在网上看到用的一个简单的方法：

class NumArray {
public:
NumArray(vector<int> nums){
re.resize(nums.size()+1);
partial_sum(nums.begin(),nums.end(),re.begin()+1);
}
int sumRange(int i, int j) {
return re[j+1]-re[i];
}
private:
vector<int>re;
};